Motor control device and image forming apparatus including the same

ABSTRACT

In a device for controlling a motor mounted on a system with different operation modes, a controller unit calculates an input value for the motor based on an output value of the motor and a target value depending on operation modes of the system, and instructs a driver circuit of the motor to apply the input value to the motor. An estimation unit estimates an amount of rise in temperature of the motor by applying the input value for the motor to a thermal model of the motor. A notification unit compares a value estimated by the estimation unit with a threshold value. When the estimated value exceeds the threshold value, the notification unit sends a request for change of operation mode to the system.

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention claims priority under 35 U.S.C. § 119 to JapaneseApplication No. 2015-016763 filed Jan. 30, 2015, the entire content ofwhich is incorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to technologies of controlling motors, andin particular, those used in image forming apparatuses.

Background

Electrophotographic image forming apparatuses such as laser printers usemotors to actuate a variety of movable members. For example, such anapparatus rotates conveyance rollers to carry a sheet within theapparatus; the apparatus rotates a photoreceptor drum to allow itscircumference surface to undergo steps of an image forming process, suchas electrostatic charge, exposure, developing, transfer, and cleaning,in turn; the apparatus rotates a developing roller to allow itscircumference surface to attract toner particles and then make themadhere onto a photoreceptor drum; the apparatus rotates an intermediatetransfer belt to allow its surface to receive a toner image from aphotoreceptor drum and then transfer it onto a sheet.

For recent image forming apparatuses, there has been developed atechnology of separating from motors their driver circuits, esp. theircontroller circuits, more specifically pre-driver integrated circuits,and of integrating them with the controller circuits of the apparatuses.This technology prevents such a motor from overheating as follows.

Since separated from the motor, its controller circuit cannot detect anactual amount of rise in temperature of the motor, i.e., a difference intemperature between the motor and its surroundings. The controllercircuit also cannot measure an amount of electric current of the motorif the circuit has no new sensor. The controller circuit thus, in orderto prevent the motor from overheating with conventional configuration,estimates an amount of rise in temperature of the motor from the inputvalue therefor. For example, a controller circuit disclosed in JP2008-012850 estimates an amount of rise in temperature of a motor fromthe number of times that a duty ratio for pulse width modulation (PWM)control, i.e., a proportion of a pulse width to a period of pulsecurrent or pulse voltage applied to the motor, exceeds a thresholdvalue. Another controller circuit disclosed in JP 2003-079186 estimatesan amount of heat generated by a motor from an amount of current of themotor. Either of these controller circuits, when having found a risk ofoverheating a motor from an estimated amount of temperature rise or heatgeneration of the motor, reduces rotation rates of the motor or enlargestime intervals of driving the motor. In addition, the controller circuitcommunicates and cooperates with the controller circuit of the apparatusto synchronize action of an object that the motor should drive withaction of other movable members. The controller circuit thus, withoutoverheating the motor, allows the apparatus to continue to process ajob.

This technology, as discussed above, never forces to cut off powersupply to a motor, even when having found a risk of overheating themotor, in contrast to overheat protection circuits embedded into generalcontroller circuits. Accordingly, the motor is never subjected to anysudden braking force, and thus, any movable member that the motor shoulddrive, such as a conveyance roller or a photoreceptor drum, is neverforced to abruptly stop moving. The technology therefore prevents theoverheat protection from jamming sheets and damaging surfaces ofphotoreceptor drums and the like.

SUMMARY OF THE INVENTION

In order to more reliably prevent motors from overheating, thetechnology of integrating the controller circuits of the motors with thecontroller circuit of the apparatus has to enhance the accuracy ofestimating an amount of rise in temperature of the motors. This is,however, difficult for the following reasons.

Like the technology disclosed in JP 2008-012850, the technology ofestimating an amount of rise in temperature of a motor from the numberof times that a duty ratio for PWM control exceeds a threshold valueinvolves neither an amount of heat dissipated from the motor kept atrest nor an amount of heat stored in the motor restarting to operate.When two or more jobs are processed intermittently, it is thus difficultto estimate amounts of rise in temperature of the motor accuratelythroughout the time period when these jobs are processed.

Like the technology disclosed in JP 2003-079186, the technology oftabulating the relationship in amount between current flowing in andheat generated out of a motor, and of using the table to estimate anamount of rise in temperature of the motor has a limit to the variety ofload fluctuations that can be tabulated. It is thus difficult for thetechnology to deal with every load fluctuation.

An object of the present invention is to solve the above-discussedtechnical problems, and in particular, to provide a motor control devicethat can enhance the accuracy of estimating an amount of rise intemperature of a motor regardless of repetition of intermittent drive ofthe motor and a variety of load fluctuations.

A device according to one aspect of the present invention is a devicefor controlling a motor mounted on a system with different operationmodes. The device comprises a controller unit configured to calculate aninput value for the motor based on an output value of the motor and atarget value depending on operation modes of the system, and to instructa driver circuit of the motor to apply the input value to the motor; anestimation unit configured to estimate an amount of rise in temperatureof the motor by applying the input value for the motor to a thermalmodel of the motor; and a notification unit configured to compare avalue estimated by the estimation unit with a threshold value, and whenthe estimated value exceeds the threshold value, to send a request forchange of operation mode to the system.

An image forming apparatus according to one aspect of the presentinvention is an apparatus for, while transferring a sheet, forming animage on the sheet. The apparatus comprises a main controller unitconfigured to assign an operation mode depending on a job received froma user; two or more motors configured to be used in transferring thesheet and forming the image on the sheet; two or more driver circuitsconfigured to supply power to respective motors of the two or moremotors; and a motor control unit including: a controller unit configuredto calculate input values for the two or more motors based on outputvalues of the two or more motors and target values depending onoperation modes of the image forming apparatus, and to instruct the twoor more driver circuits to apply the input values to the respectivemotors; an estimation unit configured to estimate amounts of rise intemperature of the two or more motors by applying the input values forthe two or more motors to a thermal model of the two or more motors; anda notification unit configured to compare a value estimated by theestimation unit with a threshold value, and when the estimated valueexceeds the threshold value, to send a request for change of operationmode to the main controller unit.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, advantages, and features of the invention willbecome apparent from the following description thereof taken inconjunction with the accompanying drawings which illustrate a specificembodiment of the invention.

In the drawings:

FIG. 1 is a schematic frontal view illustrating the configuration of animage forming apparatus according to embodiment 1 of the presentinvention;

FIG. 2 is a block diagram of the control system included in the imageforming apparatus illustrated in FIG. 1;

FIG. 3 is a block diagram of the configuration common among the driverunits illustrated in FIG. 2;

FIG. 4A is a graph illustrating relationships of the input power, outputpower, and power loss of a motor to the duty ratio of PWM control thatmakes the motor keep its rotation rate N at 2,000 rpm; FIG. 4B is agraph illustrating the torque-current characteristics of the motor; FIG.4C is a graph illustrating torque-duty characteristics of the motor whenPWM control makes the motor keep its rotation rate N at 2,000 rpm or1,000 rpm;

FIG. 5A is a block diagram of the LPF illustrated in FIG. 3; FIG. 5B isa graph illustrating a temporal change in measured values of power lossthat the LPF receives; FIG. 5C is a graph illustrating the temporalchange in outputs of the LPF, i.e. estimated amounts of rise intemperature of the motor, the change caused by the change in measuredvalues shown in FIG. 5B;

FIG. 6A is a graph illustrating torque-duty characteristics of the motorwith its rotation rate N=2,400 rpm, 1,200 rpm, or 600 rpm; FIGS. 6B, 6C,and 6D are graphs illustrating relationships of duty ratios of PWMcontrol for a motor to the input power PI, output power PO, and powerloss PL of the motor when the PWM control makes the motor keep itsrotation rate N at 2,400 rpm, 1,200 rpm, and 600 rpm, respectively;

FIG. 7 is a flowchart of motor control by the configuration illustratedin FIG. 3;

FIG. 8 is a graph illustrating relationships in PWM control of a motorbetween its duty ratios and approximate values of power loss, therelationships used by a LPF according to embodiment 2 of the presentinvention;

FIG. 9 is a block diagram of a LPF according to embodiment 3 of thepresent invention;

FIG. 10 is a flow chart of motor control according to embodiment 4 ofthe present invention;

FIG. 11 is a flow chart of motor control according to embodiment 5 ofthe present invention;

FIG. 12 is a flow chart of motor control according to embodiment 6 ofthe present invention;

FIGS. 13A and 13B are two types of the flow chart of subroutine of stepS121 illustrated in FIG. 12; and

FIG. 14 is a flow chart of motor control according to embodiment 7 ofthe present invention.

DETAILED DESCRIPTION

The following describes embodiments of the present invention withreference to the drawings.

Embodiment 1 Overview of Configuration of Image Forming Apparatus

FIG. 1 is a schematic frontal view illustrating the configuration of animage forming apparatus according to embodiment 1 of the presentinvention. This image forming apparatus 100 is a color laser printer.Elements inside the printer 100 are illustrated in FIG. 1 as if theywere viewable through the front surface of a housing of the printer 100.Referring to FIG. 1, the printer 100 includes a feeding unit 10, animaging unit 20, a fixing unit 30, and an output unit 40. These elements10-40 constitute an image forming assembly of the printer 100.

Feeding Unit

The feeding unit 10 rotates conveyance rollers 12, 13, and 14, which aredisposed along a sheet conveyance path from a feeding cassette 11 to theimaging unit 20, to feed sheets SHT piece by piece from the feedingcassette 11 to the imaging unit 20. The “sheets” indicate a thin andfilm-like or plate-like material or product, such as paper and filmresin. The sheets SHT that can be housed in the feeding cassette 11 areplain papers, wood-free papers, color copier papers or coated papers,and have the size of A3, A4, A5, or B4. The timing roller 14, which isdisposed the closest to the imaging unit 20 among the conveyancerollers, generally stops rotating even while a job is processed, androtates in response to a driving signal from a main controller unitwhich is described later. In accordance with a timing indicated by thedriving signal, the timing roller 14 sends a sheet SH2 to the imagingunit 20.

Imaging Unit

The imaging unit 20 forms a toner image on the sheet SH2 fed by thefeeding unit 10.

More specifically, each of four creation units 21Y, 21M, 21C, and 21Kfirst causes a surface of a corresponding one of the photoreceptor drums25Y, 25M, 25C, and 25K to face a corresponding charger 26 while rotatingthe photoreceptor drum. This allows the facing portions of thephotoreceptor drums to be uniformly charged.

Each of the creation units 21Y, . . . next irradiates the chargedportion of a corresponding one of the photoreceptor drums 25Y, . . .with laser light emitted by an exposure unit 27. The exposure unit 27rotates a polygon mirror 271 by a motor 272 to scan the respectivesurfaces of the photoreceptor drums 25Y, . . . with the laser light. Atthis time, the exposure unit 27 modulates an amount of the laser lightbased on tone values of colors of yellow (Y), magenta (M), cyan (C), andblack (K) represented by image data. As a result, an electrostaticlatent image representing an image of each of the colors is generated onthe surface of a corresponding one of the photoreceptor drums 25Y, . . ..

Then, each of the creation units 21Y, . . . rotates a correspondingdeveloping roller 28 to cover the surface of the developing roller 28with toner particles of a corresponding one of the Y, M, C, and K colorsand bring the surface of the developing roller 28 into contact with thesurface of a corresponding one of the photoreceptor drums 25Y, . . . .As a result, the electrostatic latent image on each of the respectivesurfaces of the photoreceptor drums 25Y, . . . is developed by tonerparticles of a corresponding color. In this way, each of respectiveimages of the four colors represented by the image data is reproduced onthe surface of a corresponding one of the four photoreceptor drums 25Y,. . . , as a toner image of a corresponding color.

The imaging unit 20 rotates primary transfer rollers 22Y, 22M, 22C, and22K and driving rollers 23L and 23R to rotate an intermediate transferbelt 23, and thus to bring the surface of the intermediate transfer belt23 into contact with the four photoreceptor drums 25Y, . . . . At thistime, the respective toner images of the four colors on the surfaces ofthe photoreceptor drums 25Y, . . . are transferred onto the sameposition on the surface of the intermediate transfer belt 23 in turnfrom the surface of the photoreceptor drums 25Y, . . . , by the actionof an electric field generated between each of the primary transferrollers 22Y, . . . and a corresponding one of the photoreceptor drums25Y, . . . . As a result, a single color toner image is formed on theposition.

The imaging unit 20 further rotates a secondary transfer roller 24 inaccordance with the driving roller 23R to bring the surface of theintermediate transfer belt 23 into contact with the sheet SH2, whichpasses through a nip between the driving roller 23R and the secondarytransfer roller 24. At this time, the color toner image on theintermediate transfer belt 23 is transferred onto the surface of thesheet SH2 by the action of an electric field generated between theintermediate transfer belt 23 and the secondary transfer roller 24.Then, the imaging unit 20 rotates the secondary transfer roller 24 tosend the sheet SH2 to the fixing unit 30.

Also, the imaging unit 20 uses drum cleaners 29 and a belt cleaner 23Cto remove toner particles remaining even after the transfer from therespective surfaces of the photoreceptor drums 25Y, . . . and theintermediate transfer belt 23. The drum cleaners 29 are each a blade ora brush that is disposed between each of the primary transfer rollers22Y, . . . and the corresponding charger 26. Immediately after each ofthe photoreceptor drums 25Y, . . . contacts the intermediate transferbelt 23, a corresponding one of the drum cleaners 29 contacts a surfaceportion of the photoreceptor drum to scrape toner particles from thesurface portion. The belt cleaner 23C is a blade or a brush that isdisposed on the upstream side with respect to the four creation units21Y, . . . in the rotation direction of the intermediate transfer belt23. After the intermediate transfer belt 23 contacts the secondarytransfer roller 24, the belt cleaner 23 contacts a surface portion ofthe intermediate transfer belt 23 to scrape toner particles from thesurface portion.

Fixing Unit

The fixing unit 30 thermally fixes the toner image on the sheet SH2,which is sent from the imaging unit 20. More specifically, the fixingunit 30 rotates a fixing roller 31 and a pressure roller 32 to cause thesheet SH2 pass through a nip therebetween. At this time, the fixingroller 31 applies heat of a heater included therein to the surface ofthe sheet SH2, and the pressure roller 32 applies pressure to the heatedportion of the surface of the sheet SH2 to press the heated portionagainst the fixing roller 31. The toner image is fixed onto the surfaceof the sheet SH2 by the action of the heat applied by the fixing roller31 and the pressure applied by the pressure roller 32. Further, thefixing unit 30 rotates the fixing roller 31 and the pressure roller 32to send the sheet SH2 to an output unit 40.

Output Unit

The output unit 40 outputs a sheet SH3, onto which the toner image isfixed, to the outside of the housing of the printer 100. Morespecifically, the sheet SH3 moves along a guide plate 41 from an upperportion of the fixing unit 30 toward an outlet 42 that is provided inthe housing of the printer 100. At this time, the output unit 40 rotatesan output roller 43, which is disposed inside the outlet 42, to send outthe sheet SH3 to the outside of the outlet 42 by using thecircumferential surface of the output roller 43. This allows the sheetSH3 to be housed in an output tray 44 that is included in the uppersurface of the printer 100.

Electronic Control System of Image Forming Apparatus

FIG. 2 is a block diagram of the configuration of the electronic controlsystem of the printer 100. In this electronic control system, referringto FIG. 2, an operation unit 50 and a main controller unit 60, as wellas the elements 10-40 of the image forming unit, are communicativelyconnected to one another via a bus 90.

Operation Unit

The operation unit 50 receives a job request and data that is aprocessing target such as an image via a user's operation orcommunication with an external electronic device, and sends them to themain controller unit 60. Referring to FIG. 2, the operation unit 50includes an operation panel 51, a memory interface (I/F) 52, and anetwork (LAN) I/F 53. The operation panel 51 includes push-buttons, atouch panel, and a display. The operation panel 51 displays, on thedisplay, a GUI screen such as an operation screen and an input screenfor various parameters. The operation panel 51 identifies a push-bottomor a position on the touch panel that the user has operated, and sendsinformation relevant to the identification as operation information tothe main controller unit 60. The memory I/F 52 includes a USB port or amemory card slot, and acquires data that is a processing target directlyfrom an external storage device such as a USB memory and a hard diskdrive (HDD) via the USB port or the memory card slot. The LAN I/F 53 hasa wired or wireless connection to an external network NTW, and receivesdata that is a processing target from other electronic device that isconnected to the network NTW.

Main Controller Unit

The main controller unit 60 is an electronic circuit mounted on a singleboard built in the printer 100. Referring to FIG. 2, the main controllerunit 60 includes a CPU 61, a RAM 62, and a ROM 63. The CPU 61 controlsother elements 10, 20, . . . , which are connected to the bus 90, inaccordance with firmware. The RAM 62 provides the CPU 61 with a workarea for the CPU 61 to execute the firmware, and stores therein datathat is a processing target, which is received by the operation unit 50.The ROM 63 includes a non-rewritable semiconductor memory device, andfurther includes a rewritable semiconductor memory device such as anEEPROM, or an HDD. The former stores therein the firmware, and thelatter provides the CPU 61 with a storage area for environmentalvariables and the like.

The main controller unit 60 first controls, by the CPU 61 executingvarious types of firmware, the other elements included in the printer100 based on the operation information received from the operation unit50. More specifically, the main controller unit 60 causes the operationunit 50 to display the operation screen to receive the user's operation.In accordance with this operation, the main controller unit 60determines an operation mode such as a running mode, a standby mode, anda sleep mode, and notifies the other elements of the determinedoperation mode by sending a driving signal to them, thus causing them toperform processing according to the operation mode.

For example, when the operation unit 50 receives a print job from theuser, the main controller unit 60 first causes the operation unit 50 totransfer image data that is a print target to the RAM 62. Next, inaccordance with a print condition indicated by the print job, the maincontroller unit 60 sends a designation of the type of sheets to be fedand its feeding timing to the feeding unit 10, provides the imaging unit20 with image data representing a toner image to be formed, and sends adesignation of the surface temperature of the fixing roller 31 to bemaintained to the fixing unit 30.

Control System of Motor

Further referring to FIG. 2, the elements 10, 20, 30, and 40 of theimage forming unit include their respective driver units 10D, 20D, 30D,and 40D for controlling motors that drive movable members to be used bythe elements, such as the conveyance rollers 12-14, intermediatetransfer belt 23, photoreceptor drums 25Y, . . . , developing rollers28, fixing roller 31, and output roller 43.

FIG. 3 is a block diagram of the configuration 300 common among thedriver units 10D-40D. Referring to FIG. 3, this configuration 300includes a motor control device 310, a driver circuit 320, a motor 330,and an encoder 340. Among these elements, ones 320, 330, and 340 otherthan the motor control device 310 are provided for each motor.

The motor control device 310 is one or more electronic circuits that aremounted on a single board built in the printer 100, such as applicationspecific integrated circuits (ASIC) or field programmable gate arrays(FPGA). The motor control device 310 communicates with the maincontroller unit 60 or the motor control device of another driver unit10D, . . . , or 40D through the bus 90 shown in FIG. 2. The motorcontrol device 310 is preferably integrated on the same board as themain controller unit 60 is. The motor control device 310 uses target andactual rotation numbers or rates of the motor 330 to calculate anaverage level of voltage to be applied to the motor 330. The motorcontrol device 310 further uses this level to perform PWM control overthe driver circuit 320.

The driver circuit 320 is an inverter with one or more switching devicessuch as power transistors (FET), and uses direct voltage received fromthe power supply unit of the printer, which is not shown in figures, toapply pulse voltages to the motor 330. The driver circuit 320, inparticular, in response to a control signal CTS from the motor controldevice 310, turns the switching devices on and off to change the widthsof the pulse voltages. The average level of the voltage applied to themotor 330 thus equals the value calculated by the motor control device310.

The motor 330 is a brushless direct-current (BLDC) motor that can rotateforward and reverse in general. The encoder 340 is of an optical ormagnetic type and mounted on the body of the motor 330. The encoder 340generates an alternate-current signal FGP of a frequency depending on anactual rotation number of the motor 330, and sends the signal to themotor control device 310.

Further referring to FIG. 3, the motor control device 310 includes acontroller unit 311, estimation unit 312, and notification unit 313.These functional units 311-313 are embedded into an electronic circuitsuch as ASIC and FPGA as dedicated hardware.

Controller Unit

The controller unit 311 uses output values of the motor 330 to perform afeedback control for calculating input values of the motor 330 to beinstructed to the driver circuit 320. As the output values of the motor330, its rotation numbers are used, and as its input values, the averagelevels of the voltage to be applied to the motor 330 are used. Referringto FIG. 3, the controller unit 311 includes a rate control unit 321, PWMunit 322, and detector unit 323.

The rate control unit 321 uses the difference between measured andtarget values of the rotation rates of the motor 330 to calculate theinput values of the motor 330. More specifically, the rate control unit321 first sets the rotation number of the motor 330 to a target value,Ntg, different according to the operation mode indicated by a drivingsignal DRV from the main controller unit 60. The rate control unit 321next uses the difference between the target value Ntg and a measuredvalue, Nms, of the rotation number of the motor 330 to calculate avoltage instruction DTY. The voltage instruction DTY specifies anaverage level of the voltage to be applied to the motor 330 as a dutyratio for the PWM control. In other words, the duty ratio, d, that thevoltage instruction DTY indicates is equal to the ratio to the powersupply voltage, Vcc, of average pulse voltage, Vav, per cycle applied tothe motor 330: d=Vav/Vcc.

The PWM unit 322 uses the input values of the motor 330 calculated bythe rate control unit 321 to perform the PWM control over the drivingcircuit 320. More specifically, the PWM unit 322 sends the controlsignal CTS to the driving circuit 320 to instruct when to turn theswitching devices on and off. The PWM unit 322 thus makes the drivingcircuit 320 adjust the duty ratio of pulse voltage to the value d thatthe voltage instruction DTY indicates. The pulse voltage is to beapplied to the motor 330, i.e. the pulse width represented by the ratioto one cycle.

The detector unit 323 monitors the output signal FGP of the encoder 340to measure from the frequency of the signal the rotation number of themotor 330. The detector unit 323 feeds back the measured value Nms tocause the rate control unit 321 to calculate the voltage instructionDTY.

Estimation Unit

The estimation unit 312 uses a thermal model of the motor 330 toestimate amounts of rise in temperature of the motor 330 from its inputvalues. The “amount of rise in temperature” or “temperature rise amount”of a motor means the difference between the temperature of the coils ofthe motor or of the switching devices of the driving circuit of themotor and the ambient air temperature of the motor or its drivingcircuit.

Referring to FIG. 3, the estimation unit 312 includes a measuring unit331 and a low-pass filter (LPF) 332. The measuring unit 331 measures apower loss of the motor 330 from the duty ratio that the voltageinstruction DTY indicates. The measured value PLS is entered to the LPF332. When detecting, from the driving signal DRV received from the maincontroller unit 60, that the duty ratio is equal to zero or the motor330 is stopping, the measuring unit 331 sets the measured value PLS tozero. The LPF 332 is a first-order lag system with a digital filter, inparticular, an infinite impulse response (IIR) one, which represents athermal model of the motor 330. The “thermal model” of a motor means theformula that represents change in temperature of the motor caused bythermal exchange between the motor and its surroundings by usingparameters that characterize the thermal exchange, such as thermalcapacitance and resistance. The LPF 332 next uses the thermal model tointegrate the values PLS of power loss of the motor 330 measured by themeasuring unit 331, thus estimating an amount of rise in temperature ofthe motor 330. The estimated amount TRS is sent to the notification unit313.

Notification Unit

The notification unit 313, whenever receiving from the estimation unit312 the estimated amount TRS of rise in temperature of the motor 330,compares the amount TRS with a threshold value. This threshold valuemeets the following condition: When the temperature rise amount of themotor 330 is equal to this threshold value or less in the environmentthat maintains the ambient temperature of the motor 330 at an acceptableupper limit, the motor 330 is usable safely without its coils and theswitching devices of the driver circuit 320 burning out. In other words,this threshold value is set to be sufficiently smaller than the amountof rise in temperature that occurs when the motor 330 truly falls intoan overheat condition. This amount is hereinafter referred to as the“upper limit of temperature rise.” In particular, the difference betweenthe threshold value and the upper limit, or a margin, is ensured to besufficiently larger than a standard error of the estimated amount TRS.The temperature at which the motor 330 and the driver circuit 320 fallinto an overheat condition seems to be constant independently of theirambient temperature, which is sufficiently lower than its acceptableupper limit in many situations. Accordingly, the upper limit oftemperature rise is, in many cases, higher than the level assumed at thesetting of this threshold value. As long as the estimated amount TRS oftemperature rise is maintained to be equal to the threshold value orless, there is no risk that an actual amount of temperature rise reachesthe upper limit, i.e. the risk that the motor 330 falls into an overheatcondition. Even if the estimated amount TRS exceeds the threshold value,at the time, there seems to still remain a difference between thetemperature rise amount of the motor 330 and its upper limit.

When the estimated amount TRS of temperature rise exceeds the thresholdvalue, the notification unit 313 sends a request RQS for change inoperation mode to the CPU 61 of the main controller unit 60. Thenotification unit 313 thus allows the main controller unit 60 to changethe operation mode before the motor 330 falls into an overheatcondition, i.e. can prevent the motor 330 from overheating.

Measurement of Power Loss of Motor from Duty Ratio

The measuring unit 331 substitutes the duty ratio, d, that the voltageinstruction DTY indicates into the following quadratic equation, Eq.(1), to calculate a measured value PL of power loss of the motor 330:

$\begin{matrix}{{{P\; L} = {{{c_{2}\lbrack N\rbrack}d^{2}} + {{c_{1}\lbrack N\rbrack}d} + {c_{0}\lbrack N\rbrack}}},{{c_{2}\lbrack N\rbrack} = \frac{Vcc}{{Kt} \cdot {A\lbrack N\rbrack}}},{{c_{1}\lbrack N\rbrack} = {\frac{- 1}{A\lbrack N\rbrack}\left( {{2\;\pi\; N} + \frac{{Vcc} \cdot {d\lbrack N\rbrack}}{Kt}} \right)}},{{c_{0}\lbrack N\rbrack} = {\frac{2\;\pi\;{N \cdot {d\lbrack N\rbrack}}}{A\lbrack N\rbrack}.}}} & (1)\end{matrix}$

The constant Vcc represents power supply voltage, and the constant Ktrepresents the torque constant of the motor 330. The parameter Nrepresents a rotation rate of the motor 330. The parameters A[N] andd[N] are functions of the rotation rate N in general and represent thetorque-duty characteristics of the motor 330.

The torque constant Kt and the parameters A[N] and d[N] are calculatedfrom design values of the motor 330 or determined by experiments.Accordingly, the coefficients c₂[N], c₁[N], and c₀[N] in Eq. (1) arecalculable when the rotation rate N of the motor 330 is determined. Themeasuring unit 331 calculates these coefficients c₂[N], . . . , from thetarget value Ntg of the rotation rate set by the rate control unit 321.

Derivation of Eq. (1)

FIG. 4A is a graph illustrating relationships of the input power PI,output power PO, and power loss PL of the motor 330 to the duty ratio ofPWM control that makes the motor keep its rotation rate N at 2,000 rpm.Referring to FIG. 4A, the input power PI is represented by a quadraticcurve of the duty ratio d, and the output power PO is by a linear curve,i.e., a straight line. Accordingly, the power loss PL, the differencebetween the powers, is represented by a quadratic curve of the dutyratio d. The following explains that the input power PI and the outputpower PO have such function forms.

FIG. 4B is a graph illustrating the torque-current characteristics ofthe motor 330. Referring to FIG. 4B, the characteristics are representedby a straight line passing through the origin since the motor 330 is ofa DC type. DC motors generally generate torques, Trq, proportional tocurrent amounts, I, flowing through their coils or the switching devicesof their driver circuits (hereinafter, abbreviated to as “coils, etc.”):Trq=Kt×I. The proportionality coefficient Kt is the torque constant Ktof the motor 330. The slope of the straight line illustrated in FIG. 4Bis equal to the reciprocal of the proportionality coefficient Kt, andthus does not depend on the rotation rate N of the motor 330.

FIG. 4C is a graph illustrating the torque-duty characteristics of themotor 330 when the PWM control makes the motor keep its rotation rate Nat 2,000 rpm or 1,000 rpm. Referring to FIG. 4C, the characteristics arerepresented by straight lines since the motor 330 is of a DC type. DCmotors generally have rotation rates N proportional to the differencebetween voltages V applied across their coils, etc. and voltage dropamounts IR due to the resistance R of the coils, etc.: N=Ki (V−IR). Theproportionality coefficient Ki is the reciprocal ratio of the inducedvoltage constant of the motor 330. On the other hand, the average pulsevoltage Vav that the motor 330 receives per PWM cycle, i.e. per cycle ofvoltage pulses that the driver circuit 320 applies to the motor 330 atthe duty ratio d, is equal to the product of the power supply voltageVcc and the duty ratio d: Vav=Vcc×d. The current amount I isproportional to the torque Trq, and thus the duty ratio d and the torqueTrq satisfy the following linear equation, Eq. (2):

$\begin{matrix}{{N = {{Ki}\left( {{{Vcc} \cdot d} - {R\frac{Trq}{Kt}}} \right)}},{{\therefore d} = {{A \cdot {Trq}} + {d\lbrack N\rbrack}}},{A = \frac{R}{{Vcc} \cdot {Kt}}},{{d\lbrack N\rbrack} = {\frac{N}{{Vcc} \cdot {Ki}}.}}} & (2)\end{matrix}$

The coefficients A and d[N] are equal to the above-defined parametersA[N] and d[N], respectively. Thus, the torque-duty characteristics ofthe motor 330 of a DC type are represented by straight lines with slopesand intercepts corresponding to the parameters A[N] and d[N],respectively. In particular, the slopes A[N] are independent of therotation rates N, but the intercepts [N] are proportional to therotation rates N.

The input power PI of the motor 330 is equal to the product of theaverage voltage Vav and the current amount I: PI=Vav×I. The averagevoltage Vav is proportional to the duty ratio d, and the current amountI is to the torque Trq; the torque Trq is represented by a linearexpression of the duty ratio d derived from Eq. (2): Trq=(d−d[N])/A.Accordingly, the input power PI is represented by the followingquadratic expression of the duty ratio d, Eq. (3):

$\begin{matrix}{{PI} = {{{{Vcc} \cdot d}\frac{Trq}{Kt}} = {\frac{Vcc}{{Kt} \cdot A}{{d\left( {d - {d\lbrack N\rbrack}} \right)}.}}}} & (3)\end{matrix}$

The output power PO of the motor 330 is equal to a rate at which themotor does work upon its load, and thus is proportional to the productof the rotation rate N and the torque Trq: PO=2πN×Trq. The torque Trq isthe linear expression of the duty ratio d derived from Eq. (2):Trq=(d−d[N])/A. Accordingly, the output power PO is represented by thefollowing linear expression of the duty ratio d, Eq. (4):

$\begin{matrix}{{PO} = {\frac{2\;\pi\; N}{A}{\left( {d - {d\lbrack N\rbrack}} \right).}}} & (4)\end{matrix}$

The expression of the power loss PL=PI−PO, Eq. (1), is derived from thedifference between Eqs. (3) and (4).

As shown in Eq. (2), the parameters A and d[N] are determined by thecoefficients Kt and Ki intrinsic to the motor 330 and the resistance Rof their coils, etc. Accordingly, values of the parameters arecalculated from the design values of the motor 330 or determined byexperiments, and then stored in a memory that is, together with themeasuring unit 331, embedded into an electronic circuit such as ASIC andFPGA. The parameter d[N] is dependent on, esp. proportional to therotation rate N of the motor 330, and thus its values stored in thememory are classified according to rotation rate of the motor 330.

Principle of Estimating Temperature Rise Amount of Motor with LPF

Configuration of LPF

FIG. 5A is a block diagram of the LPF 332. Referring to FIG. 5A, the LPF332 includes between an input terminal 501 and an output terminal 502three multipliers 510, 511, 521, two delayers 531, 532, and two adders541, 542.

The input terminal 501 samples measured values, PL[i], (the integer i=0,1, 2, . . . ) of power loss of the motor 330 from the measuring unit 331at constant intervals of time, Ts. The (i+1)-th sample PL[i] indicates ameasured value of the power loss after the elapse of a time period, iTs,the integer i times one interval Ts, from the start of the sampling.

The output terminal 502 sends estimated amounts of rise in temperatureof the motor 330 to the notification unit 313 at the same intervals Ts;the amounts Tr[i] are differences between estimated amounts Tm[i] oftemperature of the coils, etc., and their ambient temperatures Ta:Tr[i]=Tm[i]−Ta. In other words, the output terminal 502 sends the(i+1)-th estimated amount Tr[i] when the time period iTs, the integer itimes one interval Ts, has elapsed after sending the first estimatedamount Tr[0].

Whenever receiving a sample PL[i] from the input terminal 501, the firstmultiplier 510 calculates the product of the sample PL[i] and a filtercoefficient a₀, and then sends the product to the second adder 542.Whenever receiving a sample PL[i] from the input terminal 501, the firstdelayer 531 holds the sample PL[i] during the sampling interval=1 Ts,and then sends the sample PL[i] to the second multiplier 511. Wheneverreceiving from the first delayer 531 a one-interval, 1-Ts, prior samplePL[i−1], the second multiplier 511 calculates the product of the samplePL[i−1] and a second filter coefficient a₁, and then sends the productto the first adder 541.

Whenever receiving from the output terminal 502 an estimated amountTr[i] of temperature rise, the second delayer 532 holds the amount Tr[i]during the sampling interval=1 Ts, and then sends the amount Tr[i] tothe third multiplier 521. Whenever receiving from the second delayer 532a one-interval, 1-Ts, prior estimated amount Tr[i−1], the thirdmultiplier 521 calculates the product of the amount Tr[i−1] and a thirdfilter coefficient b₁, and then sends the product to the first adder541.

The first adder 541 calculates the sum of an output a₁×PL[i−1] of thesecond multiplier 511 and an output b₁×Tr[i−1] of the third multiplier521, and then sends the sum to the second adder 542. The second adder542 adds the sum a₁×PL[i−1]+b₁×Tr[i−1] to an output a₀×PL[i] of thefirst multiplier 511, and then sends to the output terminal 502 thetotal as an estimated amount Tr[i].

With the above-described configuration, the LPF 332 calculates anestimated amount Tr[i] of temperature rise when the time period iTs, theinteger i times one interval Ts, has elapsed from the start of thesampling, by substituting samples, i.e. measured values PL[· ] of thepower loss into the following equation, Eq. (5):Tr[i]=a ₀ ·PL[i]+a ₁ ·PL[i−1]+b ₁ ·Tr[i−1].  (5)

Eq. (5) is a discrete representation of the thermal model of the motor330. The three filter coefficients a₀, a₁, and b₁ are numerical valuesthat are represented by the thermal capacitance and resistance of thecoils, etc., of the motor 330, especially by the thermal response rateof the motor 330, the reciprocal ratio of the thermal time constant ofthe motor. These values are determined in advance by experiments orsimulations, and are stored in a memory that is, together with the LPF332, embedded into an electronic circuit such as ASIC and FPGA.

The LPF 332 repeats the recursive calculation by the thermal model (5)at the sampling intervals Ts to calculate the weighted sum of themeasured values PL[i] of the power loss by using the filter coefficientsa₀, a₁, and b₁ as the weights. This sum is equivalent to an integral ofthe power losses PL, and indicates the estimated amount Tr oftemperature rise.

Derivation of Eq. (5)

It is considered that, while the motor 330 operates, the coils, etc.generate heat corresponding to the power loss PL, i.e. the entirety ofremaining energy after losing from the input power PI an amount PO ofwork on the application of torque to loads. Part of this heat amount PLis stored into the coils, etc., of the motor 330 to raise theirtemperature, and the remainder is dissipated into their surroundings.The amount of heat stored into the motor 330 is represented by theproduct of the thermal capacitance Ch of the coils, etc. and aderivative of their temperature Tm with respect to time, t. The amountof heat dissipated from the motor 330 is represented by the ratio of adifference, Tr, in temperature to the thermal resistance, Rh, of thecoils, etc.; the difference Tr is one between the temperature Tm of thecoils, etc. and their ambient temperature Ta, Tr=Tm−Ta. Thus, thethermal model of the motor 330 is represented by the following equation,Eq. (6):

$\begin{matrix}{{PL} = {{{{Ch} \cdot \frac{d\;{Tm}}{d\; t}} + \frac{{Tm} - {Ta}}{Rh}} = {{{Ch} \cdot \frac{d\;{Tr}}{d\; t}} + {\frac{Tr}{Rh}.}}}} & (6)\end{matrix}$

On the other hand, change in estimated amounts Tr of temperature risewithin one sampling interval, 1 Ts, is represented by an integral of thetime derivative of the amounts Tr, dTr/dt, over the sampling interval.Approximating the integral by the sum of discrete numerical valuesresults in the following equation, Eq. (7):

$\begin{matrix}{{{{Tr}\left\lbrack {t = {i\;{Ts}}} \right\rbrack} - {{Tr}\left\lbrack {t = {\left( {i - 1} \right){Ts}}} \right\rbrack}} = {{\int_{{({i - 1})}{Ts}}^{i\;{Ts}}{\frac{d\;{Tr}}{d\; t}d\; t}} \approx {\frac{Ts}{2}{\left( {{\frac{d\;{Tr}}{d\; t}\left\lbrack {i\;{Ts}} \right\rbrack} + {\frac{d\;{Tr}}{d\; t}\left\lbrack {\left( {i - 1} \right){Ts}} \right\rbrack}} \right).}}}} & (7)\end{matrix}$

Eq. (5) is derived from Eqs. (6) and (7), as described below.Especially, identifying the values of the variables Tr and PL at thetime t=i Ts with the i-th samples Tr[i] and PL[i], respectively,represents the filter coefficients a₀, a₁, and b₁ with the followingequation, Eq. (8):

$\begin{matrix}{{{{{Tr}\lbrack i\rbrack} - {{Tr}\left\lbrack {i - 1} \right\rbrack}} = {{\frac{Ts}{2}\left( {{\frac{d\;{Tr}}{d\; t}\left\lbrack {i\;{Ts}} \right\rbrack} + {\frac{d\;{Tr}}{d\; t}\left\lbrack {\left( {i - 1} \right){Ts}} \right\rbrack}} \right)} = {{{\frac{Ts}{2}\left\{ {\frac{{PL}\lbrack i\rbrack}{Ch} - \frac{{Tr}\lbrack i\rbrack}{{Rh} \cdot {Ch}} + \left( {\frac{{PL}\left\lbrack {i - 1} \right\rbrack}{Ch} - \frac{{Tr}\left\lbrack {i - 1} \right\rbrack}{{Rh} \cdot {Ch}}} \right)} \right\}}\therefore{\left( {1 + {\frac{Ts}{2}\frac{1}{{Rh} \cdot {Ch}}}} \right){{Tr}\lbrack i\rbrack}}} = {{{{\frac{Ts}{2}\frac{1}{Ch}\left( {{{PL}\lbrack i\rbrack} + {{PL}\left\lbrack {i - 1} \right\rbrack}} \right)} + {\left( {1 - {\frac{Ts}{2}\frac{1}{{Rh} \cdot {Ch}}}} \right){{Tr}\left\lbrack {i - 1} \right\rbrack}}}\mspace{20mu}\therefore{{Tr}\lbrack i\rbrack}} = {{a_{0} \cdot {{PL}\lbrack i\rbrack}} + {a_{1} \cdot {{PL}\left\lbrack {i - 1} \right\rbrack}} + {b_{1} \cdot {{Tr}\left\lbrack {i - 1} \right\rbrack}}}}}}},} & (5) \\{\mspace{79mu}{{a_{0} = {a_{1} = {{Rh}\;\frac{\frac{\omega_{A}{Ts}}{2}}{1 + \frac{\omega_{A}{Ts}}{2}}}}},{b_{1} = \frac{1 - \frac{\omega_{A}{Ts}}{2}}{1 + \frac{\omega_{A}{Ts}}{2}}},{\omega_{A} = \frac{1}{{Rh} \cdot {Ch}}},\mspace{20mu}{\frac{\omega_{A}{Ts}}{2} = {{\tan\;\frac{\omega_{D}{Ts}}{2}} = {\tan\;{\frac{Ts}{2\;\tau}.}}}}}} & (8)\end{matrix}$

The angular frequency ω_(A) is defined as the reciprocal of the productof the thermal resistance Rh and the thermal capacitance Ch of thecoils, etc. of the motor 330, i.e. the reciprocal of the time constant,τ_(A)=Rh Ch, of the thermal model of the motor 330. The angularfrequency ω_(A) represents the thermal response rate of the motor 330,and determines the cut-off angular frequency ω_(D) of the LPF 332, i.e.the time constant τ_(D)=1/ω_(D) of the LPF 332, according to Eq. (8).

Effects of Estimation by LPF

The thermal model (5) of the motor 330 involves both heat stored in itand heat dissipated from it, as shown in Eq. (6). Even while the motor330 is at rest, the LPF 332 can thus estimate how dissipation reducesthe amount of heat stored in the motor 330 from the value before themotor 330 stops.

FIG. 5B is a graph illustrating temporal changes in measured values PLof power loss that the LPF 332 receives. Referring to FIG. 5B, the motor330 starts one continuous operation at an activation time t₀, stopsduring a rest period BRT, and then performs another continuousoperation. The measuring unit 331 measures a power loss PL of the motor330 after the activation time t₀ as follows. During both the firstoperation period DR1 and the second operation period DR2, the motor 330keeps its rotation rate to the same target value, and thus maintains thepower loss PL at the same value PLT. During the rest period BRT, on theother hand, the motor 330 stops rotating, and thus maintains the powerloss PL at zero.

FIG. 5C is a graph illustrating temporal changes in output of the LPF332, i.e. changes in estimated amount Tr of rise in temperature of themotor 330 caused by change in measured value PL shown in FIG. 5B. AsFIG. 5C shows with a solid-line graph GR1, the estimated amounts Trchange as follows. During the first operation period DR1, the motor 330keeps the power loss PL, i.e. the heat amount that the motor generates,at a constant value PLT. From the activation time t₀ to the end time t₁of the first operation period DR1, the estimated amount Tr oftemperature rise thus increases from an initial value, zero, to a peakvalue, Tr1. This increase indicates that the temperature Tm (of thecoils, etc.) of the motor 330 rises from the ambient temperature Ta by aheight Tr1. During the rest period BRT, the motor 330 does not losepower, i.e. does not generate heat, and thus the estimated amount Tr oftemperature rise reduces from the peak value Tr1 to a local minimumvalue Tr2 due to heat dissipation from the motor 330. This reductionindicates that the temperature Tm of the motor 330 drops by a heightTr1−Tr2 from the temperature Ta+Tr1 at the end time t₁ of the firstoperation period DR1. During the second operation period DR2, the motor330 again keeps the power loss PL at the constant value PLT. From thestart time t₂ of the second operation period DR2, the estimated amountTr of temperature rise thus increases from the local minimum value Tr2.This increase indicates that the temperature Tm of the motor 330 risesfrom the temperature Ta+Tr2 at the end time t₂ of the rest period BRT.

As discussed above, the estimated amount Tr of temperature riserepresented by the solid-line graph GR1 in FIG. 5C reflects thefollowing two features: 1. During the rest period BRT of the motor 330,the temperature Tm of the motor 330 drops due to heat dissipation; and2. at the time t2 when the motor 330 restarts operating, an amount ofheat stored in the motor 330 causes the difference Tr2 between thetemperature Tm of the motor 330 and the ambient temperature Ta.

On the other hand, a broken-line graph GR2 in FIG. 5C represents how theestimated amount Tr of temperature rise temporally changes afterinitialized to zero at the start time t2 of the second operation periodDR2. As the graph GR2 shows, the estimated amount Tr changes during thesecond operation period DR2 similarly to during the first operationperiod DR1. Especially, the estimated amount represented by thebroken-line graph GR2 is lower than that represented by the solid-linegraph GR1. This demonstrates that the above-mentioned two features 1 and2 that the estimation of temperature rise amount reflects enable earlierdetection, i.e. more accurate detection of risk of overheating the motor330.

Even when the motor 330 keeps its rotation rate constant, the measuredvalue PL of power loss of the motor 330 actually includes fluctuationcomponents. These components are not illustrated in FIG. 5B, but appeardue to fluctuation of power that the motor 330 consumes to keep therotation rate constant against load fluctuation. This fluctuation of thepower is fed back by the controller unit 311 to fluctuation of dutyratios d that voltage instructions DTY indicate, thus dynamicallyreflected through the fluctuation of the duty ratios d into measuredvalues PL of power loss, and further into amounts Tr of rise intemperature of the motor 330 that the LPF 332 estimates.

In this way, the amounts Tr of rise in temperature of the motor 330 thatthe LPF 332 estimates dynamically reflect fluctuation of load of themotor 330, no matter how the load fluctuates. This enables high accuracyof the estimated amounts Tr irrespective of load fluctuation.

Frequency of Estimating Amount of Rise in Temperature of Motor

The temperature of the motor 330 changes sufficiently slower than pulsevoltage that the driver circuit 320 applies to the motor 330 does.Indeed, the cycle of general pulse voltage, i.e. general PWM cycle isequal to or less than the reciprocal of an upper limit of audiblefrequency, which is nearly equal to the reciprocal of two dozen kHz, ornearly equal to 10⁻⁵ seconds. Accordingly, the cycle is negligiblyshorter than the time constant τ_(A) of the thermal model (6) of themotor 330, i.e. the reciprocal of the angular frequency ω_(A).

Thus, the measuring unit 331 calculates an average of duty ratios thatthe voltage instructions DTY indicate over each predetermined intervalof time, and uses the average to measure a power loss PL. Morespecifically, the measuring unit 331 first samples duty ratios that thevoltage instructions DTY indicate at the intervals that are each equalto an integral multiple of the PWM cycle, preferably equal to anintegral multiple of one of intervals at which the controller unit 311updates the duty ratios, or several to a dozen times as long as the PWMcycle. The measuring unit 331 next calculates an average of eachpredetermined number, two or more, of samples of the duty ratios whenstoring the number of samples, and then uses the average to measure apower loss of the motor 330.

By using the average of each two or more of the duty ratios to measure apower loss PL, the measuring unit 331 can suppress the calculationamount necessary for measuring the power loss PL. Since averages of theduty ratios vary more narrowly than the duty ratios themselves do, themeasuring unit 331 can also reduce errors of measuring the power loss.

The measuring unit 331 adjusts the intervals of sampling the duty ratiosto prevent them from equaling the fluctuation cycles of load of themotor 330, and thus protects the measuring accuracy from degrading dueto the load fluctuation. More specifically, the measuring unit 331 firstsets an initial frequency of sampling the duty ratios to a valuesufficiently higher than an assumed frequency band of the loadfluctuation. This frequency band is predicted by experiments orsimulations from the mechanical characteristics of movable members thatthe motor 330 should drive, such as conveyance rollers, and mechanismsthat transmit drive forces from the motor 330 to the movable members,such as gears and belts. The measuring unit 331 next detects an actualcycle of the load fluctuation from fluctuation of the duty ratiossampled at intervals corresponding to the initial frequency, and thenadjusts the intervals of sampling the duty ratios so that the intervalshave sufficiently large differences from any of the detected cycles andintegral multiples of them.

Protection of Motor from Overheating by Change in Operation Mode

It seems to be true that an estimated amount TRS of rise in temperatureof the motor 330 exceeding the threshold value indicates that the motor330 does not reach overheat but is at risk of it. At this time, thenotification unit 313 sends the request RQS for change in operation modeto the CPU 61 of the main controller unit 60 to have the main controllerunit 60 change operation modes in such a manner that the motor 330 canavoid overheat. More specifically, the notification unit 313 has themain controller unit 60 switch to an operation mode in which the motor330 keeps a lower rotation rate. This is for the following reason.

FIG. 6A is a graph illustrating torque-duty characteristics of the motor330 with its rotation rate N=2,400 rpm, 1,200 rpm, or 600 rpm. Referringto FIG. 6A, the characteristics are represented by straight lines sincethe motor 330 is of a DC type, like the characteristics shown in FIG.4C. As Eq. (2) represents, the slopes A of the straight lines areindependent of the rotation rates N, but the intercepts d[N] of thestraight lines are proportional to the rotation rates N.

Generally when used in the printer 100, the motor 330 should apply toload constant torque independent of its rotation rate. As FIG. 6A shows,a higher rotation rate N corresponds to a higher duty ratio d. Forexample, for the torque Trq=40 mNm, the rotation rates N=2400 rpm, 1200rpm, and 600 rpm correspond to the duty ratios d nearly equal to 82%,52%, and 38%, respectively.

FIGS. 6B, 6C, and 6D are graphs illustrating relationships of dutyratios d of PWM control for the motor 330 to the input power PI, outputpower PO, and power loss PL of the motor 330 when the PWM control makesthe motor keep its rotation rate N at 2,400 rpm, 1,200 rpm, and 600 rpm,respectively. Referring to FIGS. 6B, 6C, and 6D in order, the duty ratiod=82%, 52%, and 38% relate to the power losses PL nearly equal to 6.4 W,5.4 W, and 5.0 W, respectively.

When used in the printer 100, the motor 330 running at a higher rotationrate N results a higher power loss PL, and thus generates a largeramount of heat, as discussed above. For this reason, a lower rotationrate N of the motor 330 should reduce the amount of heat.

Suppose that an amount of rise in temperature of the motor 330 exceedingthe threshold value triggers lowering the rotation rate N of the motor330. In this case, decrease in amount of generated heat lowers the rateof rise in temperature, and then stops the rise, and finally causes thetemperature to fall. The motor 330 in an operation mode keeping a lowerrotation rate accordingly limits for a longer time the rise intemperature to an amount equal to the threshold value or less.

When the estimated amount Tr of rise in temperature of the motor 330exceeds the threshold value, the notification unit 313 sends a requestfor change of operation mode to the main controller unit 60 to have themain controller unit 60 switch to an operation mode in which the motor330 keeps a lower rotation rate. Switching to the operation mode has themotor 330 generate a smaller amount of heat, and accordingly in theoperation mode, the temperature rise keeps its amount equal to thethreshold value or less for a longer time than in the previous operationmode. This enables the printer 100 to continue operations with use ofthe motor 330.

Motor Control Procedure

FIG. 7 is a flowchart of motor control by the common configuration 300illustrated in FIG. 3. This motor control starts when the controllerunit 311 activates the motor 330 to be controlled in response topower-on of the printer 100.

In step S101, the controller unit 311 receives a driving signal DRV fromthe main controller unit 60, identifies from the signal DRV an operationmode, and then sets the rotation rate of the motor 330 to a target valueNtg different according to the operation mode. Then, the motor controlproceeds to step S102.

In step S102, the controller unit 311 performs PWM control over themotor 330. More specifically, the rate control unit 321 uses thedifference between the target value Ntg and the measured value Nms ofthe rotation rate to calculate a duty ratio, and enters the duty ratioas a voltage instruction DTY into the PWM unit 322. In response to thevoltage instruction DTY, the PWM unit 322 sends the control signal CTSto the driver circuit 320 to instruct when to turn the switching deviceson and off. In response to this control signal CTS, the driver circuit320 adjusts to the value that the voltage instruction DTY indicates theduty ratio of pulse current to be supplied to the motor 330. Thedetector unit 323 measures the rotation rate of the motor 330 from thefrequency of the output signal FGP of the encoder 340, and feeds backthe measured value Nms to the rate control unit 321. The controller unit311 repeats a series of these operations in the PWM control several to adozen times. The rate control unit 321 preferably updates the duty ratioonly when the controller unit 311 repeats the PWM control several times.Then, the motor control proceeds to step S103.

In step S103, the measuring unit 331 measures the power loss of themotor 330 from the duty ratio that the voltage instruction DTYindicates. More specifically, the measuring unit 331 substitutes theduty ratio d into Eq. (1) to calculate the measured value PL of thepower loss of the motor 330. Especially when detecting stop of the motor330, for example, from the duty ratio d equal to zero, the measuringunit 331 sets the measured value PL to zero. Then, the motor controlproceeds to step S104.

In step S104, the LPF 332 uses the thermal model (5) of the motor 330 tointegrate measured values PL of power loss of the motor 330, thusestimating an amount of rise in temperature of the motor 330. Morespecifically, the LPF 332 uses the measured value PL calculated in stepS103 to perform recursive calculation by the thermal model (5), and addsthe value PL to the sum weighted by the filter coefficients a₀, a₁, andb₁. The LPF 332 further sends this sum as an estimated amount Tr oftemperature rise to the notification unit 313. Then, the motor controlproceeds to step S105.

In step S105, the notification unit 313 compares the estimated amount Trof temperature rise with the threshold value Tth to check whether or notthe estimated amount Tr exceeds the threshold value Tth. When theestimated amount Tr is equal to the threshold value Tth or less, themotor control proceeds to step S109. When the estimated amount Trexceeds the threshold value Tth, the motor control proceeds to stepS106.

In step S106, the estimated amount Tr of temperature rise exceeds thethreshold value Tth, and thus the notification unit 313 checks whetheror not available operation modes include one in which the motor 330keeps a lower rotation rate than in a current operation mode. If such anoperation mode remains, the motor control proceeds to step S107, and ifnot, the control proceeds to step S108.

In step S107, there still remains an operation mode in which the motor330 keeps a lower rotation rate than in the current operation mode.Assume, for example, that there exist three available operation modes,which are a low rate mode ML, a middle rate mode MM, and a high ratemode MH, in order of increasing rotation rate of the motor 330, and thecurrent operation mode is the high rate mode MH. In this case, thenotification unit 313 sends a request for change of operation mode tothe main controller unit 60 to have the main controller unit 60 switchto the middle rate mode MM, for example, as an operation mode in whichthe motor 330 keeps a lower rotation rate than in the current operationmode, the high rate mode MH. Then, the motor control proceeds to stepS109.

In step S108, there remains no operation mode in which the motor 330keeps any lower rotation rate than in the current operation mode. Forexample, when the current operation mode is the low rate mode ML, thenotification unit 313 has the controller unit 311 cut off power supplyto the motor 330 by setting the duty ratio d to zero or the like, andthen sends a request for suspension of jobs to the main controller unit60. In this way, the motor control device 310 forces to stop the motor330 to prevent it from overheat. Then, the motor control ends.

In step S109, the estimated amount Tr of temperature rise is equal tothe threshold value Tth or less, and thus there is no risk ofoverheating the motor 330 even if jobs continue to be processed.Accordingly, the controller unit 311 inquires the main controller unit60 about whether or not to have an instruction to power off the printer100. If the main controller unit 60 has received the instruction, themotor control ends, and if not, the control is repeated from step S101.

The above-described motor control suppresses amounts of calculation bythe measuring unit 331 and the LPF 332 since these units calculate atintervals that are each longer than the cycle of PWM control by thecontroller unit 311. In step S102, the rate control unit 321 updates theduty ratio each time the controller unit 311 repeats PWM control severaltimes. Each time the controller unit 311 repeats PWM control several toa dozen times in step S102, the measuring unit 331 measures a value PLof power loss from an average of samples of the duty ratios in stepS103, and the LPF 332 estimates an amount Tr of rise in temperature ofthe motor 330 in step S104. Accordingly, the intervals at which themeasuring unit 331 samples the duty ratios are integral multiples of thePWM cycle, preferably integral multiples of the intervals at which thecontroller unit 311 updates the duty ratios. Furthermore, both theinterval in which the measuring unit 331 measures a value PL of powerloss and the interval in which the LPF 332 estimates an amount Tr oftemperature rise are several to a dozen times as long as the PWM cycle,i.e. not less than “the interval of sampling the duty ratios” times “thenumber of samples per average of the duty ratios (≧2).”

Advantages of Embodiment 1

The motor control device 310 according to embodiment 1 of the presentinvention, as described above, measures power loss of the motor 330 fromduty ratios d of pulse voltage that the driver circuit 320 applies tothe motor 330. The device estimates from the measured values PL[i]amounts Tr of rise in temperature of the motor 330 by using the thermalmodel (5) of it. The device feeds back to fluctuation of the duty ratiosd the state of the driver circuit 320, the state of the motor 330, andfluctuations of load of the motor. The thermal model (5) of the motor330 involves both heat stored in it and heat dissipated from it. The LPF332 can thus estimate amounts TIM of temperature rise even if themeasured values PL[i] of power loss are maintained to zero while themotor 330 is at rest. Accordingly, the motor control device 310 can,even if any load fluctuation affects the motor 330 and the printer 100repeatedly processes intermittent jobs, maintain high accuracy ofestimating amounts Tr of rise in temperature of the motor 330 throughoutthe whole period of processing the jobs.

When the estimated amount TIM of rise in temperature of the motor 330exceeds the threshold value Tth, the notification unit 313 sends arequest for change of operation mode to the main controller unit 60 tohave the main controller unit 60 switch to an operation mode in whichthe motor 330 keeps a lower rotation rate. Switching to the operationmode has the motor 330 generate a smaller amount of heat, and thus avoidthe risk of overheat. The switching also reduces the rate of temperaturerise, and thus extends the length of the time when estimated amountsTr[i] of temperature rise are equal to the threshold value or less. Forthis extended time, the motor control device 310 enables the printer 100to continue operations with use of the motor 330. In this way, the motorcontrol device 310 enables the motor 330 to continue operations withoutoverheat, and thus can maintain high levels of both reliability andproductivity for job processing.

Modifications

(A) The motor control device 310 according to embodiment 1 of thepresent invention is mounted on the color laser printer 100. The motorcontrol device according to the present invention may alternatively bemounted on image forming apparatuses including printers employinganother system such as an inkjet printer, copiers, scanners, facsimilemachines, and multifunction machines.

Further, the use of the motor control device according to the presentinvention is not limited to image forming apparatuses. In any system inwhich the temperature of the motor is estimated from an output of themotor, it is advantageous to use the motor control device according tothe present invention for its estimation processing.

(B) The functional units 311, 312, and 313 of the motor control device310 illustrated in FIG. 3 are embedded as dedicated hardware into theelectronic circuit. Alternatively, these functional units 311-313 may beembodied by a CPU built in the motor control device 310 executing adedicated firmware.

(C) The motor 330 illustrated in FIG. 3 is a BLDC motor. Alternatively,the motor control device according to the present invention is effectiveas long as the motor is a DC motor even if the motor is a brushed motor.Further alternatively, even when a motor other than a DC motor such as asynchronous motor and an induction motor is used, it is effective toestimate the amount of rise in temperature of the motor by the LPFaccording to the present invention as long as power loss can beestimated.

(D) The measuring unit 331 calculates the coefficients c₂[N], . . . inEq. (1), the relational equation between the duty ratio d for PWMcontrol and the measured value PL of power loss of the motor 330, fromthe target value Ntg of the rotation rate set by the rate control unit321. Alternatively, the measuring unit 331 may calculate thesecoefficients c₂[N], . . . from the value Nms of the rotation ratemeasured by the detector unit 323. The correspondence relationshipbetween these coefficients c₂[N], . . . and the rotation rate N of themotor may be in advance tabulated and stored in a memory that is,together with the measuring unit 331, embedded into an electroniccircuit.

Regarding the parameter d[N] in Eq. (1), only a value d[N0] at aspecific rotation rate N0 may be stored in the memory. In this case, themeasuring unit 331 may calculate a value d[N=Ntg, Nms] at the targetvalue Ntg or the measured value Nms relative to the specific rotationrate N0 from the value d[N0] and a ratio of the target value Ntg or themeasured value Nms to the specific rotation rate N0.

(E) The measuring unit 331 averages the duty ratios that the voltageinstruction DTY indicates at predetermined time intervals to estimatepower loss. Alternatively, the measuring unit may use the sum of theduty ratios at predetermined time intervals to estimate power loss.Further alternatively, when the load fluctuation is sufficiently slowand fluctuation of the measured value of power loss caused by the loadfluctuation is within an acceptable error range, the measuring unit mayuse the duty ratio independently to measure a power loss withoutaveraging or summing the duty ratios. Yet alternatively, the measuringunit may first calculate the measured value of power loss from the dutyratio, store therein a plurality of calculation results, and then enterthe average value or the sum of the calculation results to the LPF 332.

(F) The LPF 332 calculates the estimated amount Tr of rise intemperature by using the thermal model (5). This estimated amount Trcorresponds to the amount of rise in temperature. However, the output ofthe LPF only needs to be a linear function, i.e. the sum of a termproportional to the amount of rise in temperature and a constant term.In this case, the amount of rise in temperature is easily converted fromthe output of the LPF. Conversely, a threshold value of the output isalso easily converted from the threshold value of the amount of rise intemperature.

For example, the output of the LPF may be represented by, instead of theamount Tr of rise in temperature, a ratio Tr/Rh of the amount Tr of risein temperature to the thermal resistance Rh. In this case, as clear fromEq. (8), the first filter coefficient a₀ and the second filtercoefficient a₁ are each a dimensionless quantity and a positive realnumber less than one, like the third falter coefficient b₁. Thus,calculation of these coefficients is easily simplified for example owingto availability of the fixed point arithmetic.

(G) The notification unit 313 regards the estimated amount of rise intemperature of the motor as the difference between the acceptable upperlimit of the ambient temperature of the motor and the temperature of themotor itself. Alternatively, when a temperature sensor is mounted on theprinter, the notification unit may estimate the temperature of the motoritself from the sum of a value measured by the temperature sensor andthe estimated amount of rise in temperature. In this case, differentthreshold values of the amount of rise in temperature can be set fordifferent ambient temperatures, and thus the accuracy of having found arisk of overheating of the motor can be further enhanced. Especially,the frequency of reducing the rotation rate of the motor can be reducedwithout overheating the motor, and thus high levels of both theproductivity and the operability of the printer can be maintained.

(H) When requesting the main controller unit 60 to change operationmodes in response to exceeding of the threshold value by the estimatedamount of rise in temperature, the notification unit 313 may alsorequest the main controller unit 60 to notify a user of the change forexample by display on the operation panel 51.

The notification unit 313 may cause the main controller unit 60 to storea history relating to the exceeding of the threshold value by theestimated amount of rise in temperature, in the ROM 63 of the maincontroller unit 60 or an external storage device via the memory I/F 52.This history is advantageous for use in maintenance of the printer 100.Further, whenever the estimated amount of rise in temperature exceedsthe threshold value, the notification unit 313 may cause the maincontroller unit 60 to report the exceeding to a server or the like on anexternal network NTW via the LAN I/F 53. Alternatively, a historyrelating to the exceeding may be periodically reported.

Embodiment 2

A motor control device according to embodiment 2 of the presentinvention is mounted on a color laser printer, like the motor controldevice 310 according to embodiment 1. This motor control device differsfrom the motor control device 310 according to embodiment 1 only inconfiguration of the estimation unit, and includes other elementssimilar to those in embodiment 1. Accordingly, the following explainsonly the difference in the estimation unit, and incorporates theexplanation of embodiment 1 for the similar elements.

The estimation unit 312 according to embodiment 1 has the measuring unit331 measure power loss of the motor 330 from the duty ratios dcontrolled by the PWM unit 321, and has the LPF 332 integrate themeasured values PL. In contrast, an estimation unit according toembodiment 2 has an LPF integrate the duty ratios themselves withoutusing the measuring unit, as explained below.

FIG. 8 is a graph illustrating relationships in PWM control between theduty ratios d and approximate values PM of power loss of the motor 330.The relationships are used by the LPF according to embodiment 2.Referring to FIG. 8, the relationships are represented by a straightline LT. The straight line LT is tangent at a point PT to a quadraticcurve (see Eq. (1)) that more accurately represents the power loss PL.In other words, the duty ratios d and the approximate values PM of powerloss of the motor satisfy the following equation, Eq. (9):PM=(2c ₂[N]d₀ +c ₁[N])(d−d ₀)+PL ₀ =C ₁ d+C ₀ ,C ₁=2c ₂[N]d₀ +c₁[N],C₀=(2c ₂[N]d₀ +c ₁[N])d ₀ +PL ₀.  (9)

The constants d₀ and PL₀ represent the duty ratio and power loss at thetangent point PT, respectively. The coefficients c₂[N] and c₁[N] areequal to those in Eq. (1).

The LPF uses, instead of Eq. (5), the following equation, Eq. (10), asthe thermal model of the motor to estimate an amount Tr_(m)[i] of risein temperature of the motor from a sample d[i] of the duty ratio:Tr _(m) =a _(m0) ·d[i]+a_(m1) ·d[i−1]+b ₁ ·Tr _(m)[i−1].  (10)

The third filter coefficient b₁ is equal to that in Eq. (5). The firstfilter coefficient a_(m0), the second filter coefficient a_(m1), and theamount Tr_(m)[i] of temperature rise relate to the filter coefficientsand amount Tr[i] of temperature rise in Eq. (5), and the coefficients C₁and C₀ in Eq. (9) as represented by the following equation, Eq. (11):a _(m0) =a _(m1) =a ₀ ·C ₁,(1−b ₁)Tr ₀=2a ₀ ·C ₀ ,Tr _(m)[i]=Tr[i]−Tr₀  (11)

As is clear from FIG. 8, deviation of approximate values PM from moreaccurate measured values PL is tiny in the vicinity of the tangent pointPT. Accordingly, the LPF sets the duty ratio d₀ of the tangent point PTto a value that the driver circuit most frequently uses in the normaloperations, 60% in the example shown in FIG. 8. When load fluctuation issufficiently small to cause duty ratios d to slightly deviate from thevalue d₀ (for example, 60%) of the tangent point PT, temperature riseamounts Tr_(m)[i] estimated by Eq. (10) are correct to a high level ofaccuracy that allows the estimated amounts to be used for protection ofthe motor from overheat.

Advantages of Embodiment 2

The motor control device according to embodiment 2 of the presentinvention, as discussed above, estimates amounts of rise in temperatureof the motor by using the thermal model of the motor, like the motorcontrol device 310 according to embodiment 1. Accordingly, the motorcontrol device has high accuracy of estimating amounts of temperaturerise. This enables the motor to continue operation without overheat. Asa result, the motor control device can maintain high levels of bothreliability and productivity for job processing.

Furthermore, the estimation unit according to embodiment 2 has the LPFuse the thermal model (10) to integrate the duty ratios d, as discussedabove. This can omit calculating power loss of the motor from the dutyratios by Eq. (1), i.e., can eliminate the measuring unit 331, incontrast to the estimation unit 312 according to embodiment 1. As aresult, the estimation unit can improve the simplicity of processing andconfiguration necessary for estimating amounts of temperature rise.

Embodiment 3

A motor control device according to embodiment 3 of the presentinvention is mounted on a color laser printer, like the motor controldevice 310 according to embodiment 1. This motor control device differsfrom the motor control device 310 according to embodiment 1 only inconfiguration of the LPF, and includes other elements similar to thosein embodiment 1. Accordingly, the following explains only the differencein the LPF, and incorporates the explanation of embodiment 1 for thesimilar elements.

The LPF 332 according to embodiment 1 uses the filter coefficients a₀,a₁, and b₁ in Eq. (8) both while the motor 330 operates and while it isat rest. This is because the motor 330 is approximated both for anoperating period and for a rest period by the thermal model (6) to asufficiently high level of accuracy, and especially the motor for eachperiod has a time constant τ_(A), i.e. the product of a thermalresistance Rh and a thermal capacitance Ch, τ_(A)=RhCh=1/ω_(A),sufficiently close to the time constant of the thermal model. Incontrast, an LPF according to embodiment 3 uses different thermalmodels, especially different filter coefficients, while the motor 330operates and while it is at rest; the difference between the thermalmodels corresponds to the difference in rate of thermal response of themotor 330 between during operation and during rest, as explained below.

FIG. 9 is a block diagram of an LPF 900 according to embodiment 3.Referring to FIG. 9, this LPF 900, in contrast to the LPF 332illustrated in FIG. 5A, doubles the number of each multiplier andincludes three additional switches 941, 942, and 943. Other elementsillustrated in FIG. 9 are similar to those illustrated in FIG. 5A.Accordingly, for these elements, the same reference numerals as in FIG.5A are assigned and the explanation of FIG. 5A is incorporated.

A first main multiplier 911 and a first sub multiplier 912 differ fromeach other only in first filter coefficient, a₀₁ and a₀₂. Wheneverreceiving a sample PL[i] from the input terminal 501, the multipliers911 and 912 calculate the products of the sample and their respectivefirst filter coefficients a₀₁ and a₀₂, and send the products to thefirst switch 941.

A second main multiplier 921 and a second sub multiplier 922 differ fromeach other only in second filter coefficient, a₁₁ and a₁₂. Wheneverreceiving a one-interval, 1-Ts, prior sample PL[i−1] from the firstdelayer 531, the multipliers 921 and 922 calculate the products of thesample and their respective second filter coefficients a₁₁ and a₁₂, andsend the products to the second switch 942.

A third main multiplier 931 and a third sub multiplier 932 differ fromeach other only in third filter coefficient, b₁₁ and b₁₂. Wheneverreceiving a one-interval, 1-Ts, prior estimated amount Tr[i−1] from thesecond delayer 532, the multipliers 931 and 932 calculate the productsof the amount and their respective third filter coefficients b₁₁ andb₁₂, and send the products to the third switch 943.

The three switches 941, 942, and 943 each distinguish between theoperating period and the rest period of the motor by checking whether ornot the value of power loss of the motor measured by the measuring unitis equal to zero. The switches 941, . . . select outputs of the mainmultipliers 911, 921, and 931 during the operating period, and outputsof the sub multipliers 912, 922, and 932 during the rest period.Furthermore, the first switch 941 sends the selected output to thesecond adder 542, and the second switch 942 and the third switch 943both send the selected outputs to the first adder 541.

With the above-described configuration, the LPF 900 estimates amountsTr[i] of temperature rise by using different thermal models during theoperating period and during the rest period. The first thermal model forthe operating period, heating model, and the second one for the restperiod, cooling model, are represented by the following equations, Eqs.(12) and (13), respectively:Tr[i]=a ₀₁ ·PL[i]+a ₁₁ ·PL[i−1]+·Tr[i−1],  (12)Tr[i]=a₀₂ ·PL[i]+a ₁₂ ·PL[i−1]+b ₁₂ ·Tr[i−1].  (13)

The filter coefficients a₀₁, a₁₁, and b₁₁ of the heating model (12) aredetermined from the time constant τ_(A1) of thermal response during theoperating period of the motor; the filter coefficients a₀₂, a₁₂, and b₁₂of the cooling model (13) are determined from the time constant τ_(A2)of thermal response during the rest period of the motor; those arerepresented by the following equations, Eq. (14):

$\begin{matrix}{{a_{01} = {a_{11} = {{Rh}_{1}\frac{\frac{Ts}{2\;\tau_{A\; 1}}}{1 + \frac{Ts}{2\;\tau_{A\; 1}}}}}},{b_{11} = \frac{1 - \frac{Ts}{2\;\tau_{A\; 1}}}{1 + \frac{Ts}{2\;\tau_{A\; 1}}}},{\tau_{A\; 1} = {{Rh}_{1} \cdot {Ch}_{1}}},{\frac{Ts}{2\;\tau_{A\; 1}} = {\tan\;\frac{Ts}{2\;\tau_{D\; 1}}}},{a_{02} = {a_{12} = {{Rh}_{2}\frac{\frac{Ts}{2\;\tau_{A\; 2}}}{1 + \frac{Ts}{2\;\tau_{A\; 2}}}}}},{b_{12} = \frac{1 - \frac{Ts}{2\;\tau_{A\; 2}}}{1 + \frac{Ts}{2\;\tau_{A\; 2}}}},{\tau_{A\; 2} = {{Rh}_{2} \cdot {Ch}_{2}}},{\frac{Ts}{2\;\tau_{A\; 2}} = {\tan\;\frac{Ts}{2\;\tau_{D\; 2}}}},} & (14)\end{matrix}$

The constants Rh₁ and Ch₁ represent the thermal resistance and thermalcapacitance for the operating period of the motor, respectively. Theconstants Rh₂ and Ch₂ represent the thermal resistance and thermalcapacitance for the rest period of the motor, respectively. Theconstants τ_(D1) and τ_(D2) represent the time constants of the LPF 900for the operating period and for the rest period, respectively.

Advantages of Embodiment 3

The motor control device according to embodiment 3 of the presentinvention, as discussed above, uses the two different thermal models(12) and (13) for estimating amounts of rise in temperature of the motorwhile it operates and while it is at rest. This enables the motorcontrol device, even if processing intermittent jobs makes the motor 330frequently repeat operating and stopping, to maintain high accuracy ofestimating amounts of rise in temperature of the motor throughout thewhole period of processing the jobs. In this way, the motor controldevice enables the motor to continue operations without overheat, andthus can maintain high levels of both reliability and productivity forjob processing.

Embodiment 4

A motor control device according to embodiment 4 of the presentinvention is mounted on a color laser printer, like the motor controldevice 310 according to embodiment 1. This motor control device differsfrom the motor control device 310 according to embodiment 1 only in partof the motor control procedure, and includes other elements similar tothose in embodiment 1. Accordingly, the following explains only thedifference in the flow of the control, and incorporates the explanationof embodiment 1 for the similar elements.

The motor control according to embodiment 1, as illustrated in FIG. 7,when the notification unit 313 confirms in step S105 that an estimatedamount of temperature rise exceeds the threshold value, immediatelyproceeds to step S106. Then, the notification unit 313 checks whether ornot there exists an operation mode in which the motor 330 keeps a lowerrotation rate than in the current operation mode. In contrast, the motorcontrol according to embodiment 4, even when an estimated amount oftemperature rise exceeds the threshold value, as long as time shorterthan a certain length remains before processing jobs is finished, hasthe motor continue operations without having the notification unit sendany request for change of operation mode; the certain length of time ispredicted to elapse before the motor actually overheats.

FIG. 10 is a flow chart of the motor control according to embodiment 4.This procedure differs from that illustrated in FIG. 7 only in step S110inserted between step S105 and step S106. Other steps are similar tothose in embodiment 1. Accordingly, the following explains details ofthe different step S110, and incorporates the explanation of embodiment1 for details of the similar steps.

In step S101, the rotation rate of the motor 330 is set to a targetvalue Ntg depending on operation modes. Then, the motor control proceedsto step S102.

In step S102, PWM control over the motor 330 is repeated several to adozen times. Then, the motor control proceeds to step S103.

In step S103, power loss of the motor 330 is measured from the dutyratio that the voltage instruction DTY indicates. For example, when stopof the motor 330 is detected from the duty ratio equal to zero, themeasured value is set to zero. Then, the motor control proceeds to stepS104.

In step S104, the LPF 332 uses the thermal model (5) of the motor 330 toestimate an amount of rise in temperature of the motor 330. Then, themotor control proceeds to step S105.

In step S105, the estimated amount Tr of temperature rise is checkedwhether or not to exceed the threshold value Tth. When the estimatedamount Tr is equal to the threshold value Tth or less, the motor controlproceeds to step S109. When the estimated amount Tr exceeds thethreshold value Tth, the motor control proceeds to step S110.

In step S110, the estimated amount Tr of temperature rise exceeds thethreshold value Tth, and then the notification unit 313 inquires themain controller unit 60 to check whether or not time shorter than anacceptable upper limit remains before processing jobs is finished. The“acceptable upper limit” is the sum of a certain time and a margin. Thecertain time is predicted to elapse from when the estimated amount Tr oftemperature rise exceeds the threshold value Tth until when the motor330 actually overheats. The margin is based on estimation errors.

In response to the inquiry from the notification unit 313, the maincontroller unit 60 first counts how many sheets now remain unprintedamong the number of sheets required by current jobs. From the number ofunprinted sheets and current processing speeds, the main controller unit60 next predicts the length of remaining time before processing the jobsis finished, and then returns the predicted length to the notificationunit 313. When the length is shorter than the acceptable upper limit,the motor control proceeds to step S109. When the length is equal to theacceptable upper limit or more, the motor control proceeds to step S106.

When the processing speeds are independent of operation modes, thenotification unit 313 may convert the acceptable upper limit into thenumber of sheets printable at the processing speeds, and then comparethe number of printable sheets with the number of unprinted sheets.

In step S106, time equal to the acceptable upper limit or more remainsbefore processing the jobs is finished, and thus continuation ofprocessing the jobs might cause the motor 330 to overheat beforeprocessing the jobs is finished. Accordingly, the notification unit 313checks whether or not there exists an operation mode in which the motor330 keeps a lower rotation rate than in the current operation mode. Ifsuch an operation mode exists, the motor control proceeds to step S107.If not, the control proceeds to step S108.

In step S107, there still remains an operation mode in which the motor330 keeps a lower rotation rate than in the current operation mode, andthus the notification unit 313 requests the main controller unit 60 toswitch to the remaining operation mode. Then, the motor control proceedsto step S109.

In step S108, there remains no operation mode in which the motor 330keeps a lower rotation rate than in the current operation mode, and thusthe notification unit 313 has the controller unit 311 stop the motor330, and has the main controller unit 60 suspend processing the jobs. Inthis way, the motor 330 is prevented from overheat. Then, the motorcontrol ends.

In step S109, the estimated amount Tr of temperature rise is equal tothe threshold value Tth or less, or alternatively time shorter than theacceptable upper limit remains before processing the jobs is finished.Accordingly, continuation of processing the jobs has no risk ofoverheating the motor 330, or processing the jobs will be finishedbefore the motor 330 overheats. Thus, the controller unit 311 checkswhether or not it has received an instruction to power off the printer100. If the controller unit 311 has received the instruction, the motorcontrol ends. If not, the control is repeated from step S101.

Advantages of Embodiment 4

The motor control device according to embodiment 4 of the presentinvention estimates amounts of rise in temperature of the motor by usingthe thermal model of the motor, like the motor control device 310according to embodiment 1. Accordingly, the motor control device hashigh accuracy of estimating amounts of temperature rise. This enablesthe motor to continue operation without overheat. As a result, the motorcontrol device can maintain high levels of both reliability andproductivity for job processing.

Especially even if an estimated amount of temperature rise exceeds thethreshold value, as long as remaining time before the jobs are finishedis shorter than the acceptable upper limit, the motor control deviceaccording to embodiment 4 has the motor continue operation withoutsending any request for change of operation mode. In other words, theprinter can finish processing the jobs before the motor overheats, andthus the motor control device has the printer continue to process thejobs without change of operation mode. This avoids delay of processingthe jobs due to change of operation mode without causing the motor tooverheat, and thus the motor control device can maintain higherproductivity.

Embodiment 5

A motor control device according to embodiment 5 of the presentinvention is mounted on a color laser printer, like the motor controldevice 310 according to embodiment 1. This motor control device differsfrom the motor control device 310 according to embodiment 1 only in partof the motor control procedure, and includes other elements similar tothose in embodiment 1. Accordingly, the following explains only thedifferent part of the motor control procedure, and incorporates theexplanation of embodiment 1 for the similar elements.

The motor control according to embodiment 1, when an estimated amount oftemperature rise exceeds the threshold value, requests change ofoperation mode to reduce the rotation rate of the motor. In contrast,the motor control according to embodiment 5 requests change of operationmode to enlarge intervals of feeding sheets.

Generally, longer intervals of feeding sheets put lighter average loadson the motors. For example, the conveyance rollers are not in contactwith sheets for longer time. Only when the conveyance rollers are incontact with sheets, the motors driving the conveyance rollers are underheavy load. Thus, lighter time-averaged load puts on the motors. Asillustrated in FIG. 6, the motors generating lower torque lose lesspower. Therefore, the lighter average load indicates smaller averageamounts of heat that the motors generate.

The motor control device according to embodiment 5 accordingly, when anestimated amount of temperature rise exceeds the threshold value,changes the current operation mode to an operation mode in which sheetsare fed at longer intervals than in the current operation mode. Thisreduces the average amounts of heat that the motors generate, and thusreduces the rates of rise in temperature of the motors.

FIG. 11 is a flow chart of motor control according to embodiment 5. Thisprocedure differs from that illustrated in FIG. 7 only in steps S116 andS117 instead of steps S106 and S107, respectively. Other steps in thisprocedure are similar to those in embodiment 1. Accordingly, thefollowing explains details of only steps S116 and S117, and incorporatesthe explanation of embodiment 1 for details of the similar steps.

In step S101, the rotation rate of the motor 330 is set to a targetvalue Ntg depending on operation modes. Then, the motor control proceedsto step S102.

In step S102, PWM control over the motor 330 is repeated several to adozen times. Then, the motor control proceeds to step S103.

In step S103, power loss of the motor 330 is measured from the dutyratio that the voltage instruction DTY indicates. For example, when stopof the motor 330 is detected from the duty ratio equal to zero, themeasured value is set to zero. Then, the motor control proceeds to stepS104.

In step S104, the LPF 332 uses the thermal model (5) of the motor 330 toestimate the amount of rise in temperature of the motor 330. Then, themotor control proceeds to step S105.

In step S105, the estimated amount Tr of temperature rise is checkedwhether or not to exceed the threshold value Tth. When the estimatedamount Tr is equal to the threshold value Tth or less, the motor controlproceeds to step S109. When the estimated amount Tr exceeds thethreshold value Tth, the motor control proceeds to step S116.

In step S116, the estimated amount Tr of temperature rise exceeds thethreshold value Tth, and then the notification unit 313 checks whetheror not available operation modes include one in which sheets are fed atlonger intervals than in the current operation mode. If such anoperation mode exists, the motor control proceeds to step S117. If not,the motor control proceeds to step S108.

In step S117, there still remains an operation mode in which sheets arefed at longer intervals than in the current operation mode. Accordingly,the notification unit 313 sends a request for change of operation modeto the main controller unit 60 to have the main controller unit 60switch to the operation mode in which sheets are fed at longer intervalsthan in the current operation mode. Then, the motor control proceeds tostep S109.

In step S108, there remains no operation mode in which sheets are fed atlonger intervals than in the current operation mode. Accordingly, thenotification unit 313 has the controller unit 311 stop the motor 330,and has the main controller unit 60 suspend processing the jobs. In thisway, the motor 330 is prevented from overheat. Then, the motor controlends.

In step S109, the estimated amount Tr of temperature rise is equal tothe threshold value Tth or less, or alternatively time shorter than theacceptable upper limit remains before processing the job is finished.Accordingly, continuation of processing the jobs has no risk ofoverheating the motor 330, or processing the jobs will be finishedbefore the motor 330 overheats. Thus, the controller unit 311 checkswhether or not it has received an instruction to power off the printer100. If the controller unit 311 has received the instruction, the motorcontrol ends. If not, the control is repeated from step S101.

Advantages of Embodiment 5

The motor control device according to embodiment 5 of the presentinvention estimates amounts of rise in temperature of the motor by usingthe thermal model of the motor, like the motor control device 310according to embodiment 1. Accordingly, the motor control device hashigh accuracy of estimating amounts of temperature rise.

When an estimated amount of temperature rise exceeds the thresholdvalue, the notification unit 313 requests the main control unit toswitch to an operation mode in which the intervals of feeding sheets arelonger. Switching to the operation mode has the motor generate a smalleramount of heat, and thus avoid the risk of overheat. Furthermore,enlargement of the intervals of feeding sheets lowers the rate of thetemperature rise, and thus extends the length of the time when estimatedamounts of the temperature rise are equal to the threshold value orless. For this extended time, the motor control device enables theprinter to continue operations with use of the motor. In this way, themotor control device enables the motor to continue operations withoutoverheat, and thus can maintain high levels of both reliability andproductivity for job processing.

Embodiment 6

A motor control device according to embodiment 6 of the presentinvention is mounted on a color laser printer, like the motor controldevice 310 according to embodiment 1. This motor control device differsfrom the motor control device 310 according to embodiment 1 only in partof motor control procedure, and includes other elements similar to thosein embodiment 1. Accordingly, the following explains only the differentpart of the motor control procedure, and incorporates the explanation ofembodiment 1 for the similar elements.

The motor control according to embodiment 1, as illustrated in FIG. 7,when there exists no operation mode in which the motor keeps a lowerrotation rate than in the current operation mode, immediately suspendsprocessing the jobs and then ends processing. In contrast, the motorcontrol according to embodiment 6 continues processing even after theprinter suspends processing the jobs. When a predetermined length oftime has elapsed after the printer suspends processing the jobs, or whenan estimated amount of temperature rise drops to the threshold value orless, the motor control has the printer restart processing the jobs.

FIG. 12 is a flow chart of the motor control according to embodiment 6.This procedure differs from that illustrated in FIG. 7 only in step S121inserted between step S108 and step S109. Other steps in this procedureare similar to those in embodiment 1. Accordingly, the followingexplains details of only step S121, and incorporates the explanation ofembodiment 1 for details of the similar steps.

In step S101, the rotation rate of the motor 330 is set to a targetvalue Ntg depending on operation modes. Then, the motor control proceedsto step S102.

In step S102, PWM control over the motor 330 is repeated several to adozen times. Then, the motor control proceeds to step S103.

In step S103, power loss of the motor 330 is measured from the dutyratio that the voltage instruction DTY indicates. For example, when stopof the motor 330 is detected from the duty ratio equal to zero, themeasured value is set to zero. Then, the motor control proceeds to stepS104.

In step S104, the LPF 332 uses the thermal model (5) of the motor 330 toestimate an amount of rise in temperature of the motor 330. Then, themotor control proceeds to step S105.

In step S105, the estimated amount Tr of temperature rise is checkedwhether or not to exceed the threshold value Tth. When the estimatedamount Tr is equal to the threshold value Tth or less, the motor controlproceeds to step S109. When the estimated amount Tr exceeds thethreshold value Tth, the motor control proceeds to step S106.

In step S106, the estimated amount Tr of temperature rise exceeds thethreshold value Tth, and thus the notification unit 313 checks whetheror not there exists an operation mode in which the motor 330 keeps alower rotation rate than in the current operation mode. If such anoperation mode exists, the motor control proceeds to step S107. If not,the control proceeds to step S108.

In step S107, there still remains an operation mode in which the motor330 keeps a lower rotation rate than in the current operation mode, andthus the notification unit 313 requests the main controller unit 60 toswitch to the remaining operation mode. Then, the motor control proceedsto step S109.

In step S108, there remains no operation mode in which the motor 330keeps a lower rotation rate than in the current operation mode, and thusthe notification unit 313 has the controller unit 311 cut off powersupply to the motor 330, and has the main controller unit 60 suspendprocessing the jobs. In this way, the motor 330 is prevented fromoverheat. Then, the motor control proceeds to step S121.

In step S121, the notification unit 313 judges whether the jobprocessing suspended in step S108 is to be restarted. There are the twofollowing conditions for restarting the job processing: (A) the lengthof time during which the job processing is suspended reaches apredetermined value; and (B) an estimated amount of temperature risedrops to the threshold value or less during the time. When either of theconditions is satisfied, the notification unit 313 sends a request forrestart of the job processing to the main controller unit 60. Then, themotor control proceeds to step S109.

In step S109, the estimated amount Tr of temperature rise is equal tothe threshold value Tth or less, and thus there is no risk ofoverheating the motor 330 even if the jobs continue to be processed.Thus, the controller unit 311 checks whether or not it has received aninstruction to power off the printer 100. When the controller unit 311has received the instruction, the motor control ends. If not, thecontrol is repeated from step S101.

FIG. 13A is a flow chart of subroutine of step S121 illustrated in FIG.12 when condition (A) is adopted as condition for restarting jobprocessing: “the length of time during which the job processing issuspended reaches the predetermined value.”

In step SA1, the notification unit 313 measures the length of time thatelapses after the suspension of job processing, and checks whether ornot the elapsed time exceeds a predetermined value Twt. This value Twtrepresents the length of time necessary for the motors overheating to socool to resume operation. For example, the value Twt is set to thelength of time necessary for an amount of rise in temperature of themotor to drop from the threshold value Tth to zero within an acceptableerror range, i.e. for the temperature of the motor to substantially dropto the ambient temperature. When the elapsed time exceeds thepredetermined value Twt, the motor control proceeds to step SA2. Whenthe elapsed time is equal to the predetermined value or less, thecontrol repeats step SA1.

In step SA2, the elapsed time from the suspension of job processingexceeds the predetermined value Twt. Accordingly, the motor is regardedas having cooled to resume, and thus the notification unit 313 sends arequest for restart of jobs to the main controller unit 60. Then, themotor control returns from the subroutine to step S109 illustrated inFIG. 12.

FIG. 13B is a flow chart of subroutine of step S121 illustrated in FIG.12 when condition (B) is adopted as condition for restarting jobprocessing: “an estimated amount of temperature rise drops to thethreshold value or less during suspension of job processing.”

In step SB1, the notification unit 313 sets a new threshold value Tntfor amount of rise in temperature of the motor a value at which themotor stopped in step S108 can resume. This new threshold value Tnt isequal to or less than the threshold value Tth at the suspension of jobprocessing, and for example satisfies the following condition: “Even ifprocessing a job is restarted, amounts of rise in temperature of themotor increase from the new threshold value Tnt to at most the thresholdvalue Tth at the suspension of job processing, during the period afterprocessing the job is restarted until it is finished.” Then, the motorcontrol proceeds to step SB2.

In step SB2, the LPF 332 uses the thermal model (5) of the motor toestimate amounts of rise in temperature of the motor. In other words,the LPF 332 integrates the input values PL[·]=0 for the thermal model atthe sampling intervals Ts, and sends an integrated result as anestimated amount Tr of temperature rise to the notification unit 313.The sampling intervals Ts are set to values at which samples aretraceable with a sufficient precision to fluctuations of amounts oftemperature rise due to heat dissipation from the motor; the values arefor example comparable with the time constant τ_(A)=R_(h)C_(h) ofthermal response of the motor. Then, the motor control proceeds to stepSB3.

In step SB3, the notification unit 313 compares the estimated amount Trof temperature rise with the new threshold value Tnt to check whetherthe estimated amount Tr is equal to the threshold value Tnt or less.When the estimated amount Tr is equal to the threshold value Tnt orless, the motor control proceeds to step SB4. When the estimated amountTr exceeds the threshold value Tnt, the control is repeated from stepSB2.

In step SB4, the estimated amount Tr is equal to the threshold value Tntor less, and thus, even if processing a job is restarted, until it isfinished, there is no risk that the temperature rise increases so largethat the motor needs to be stopped. Accordingly, the notification unit313 sends a request for restart of processing jobs to the maincontroller unit 60. Then, the motor control returns from the subroutineto step S109 illustrated in FIG. 12.

Advantages of Embodiment 6

The motor control device according to embodiment 6 of the presentinvention estimates amounts of rise in temperature of the motor by usingthe thermal model of the motor, like the motor control device 310according to embodiment 1. Accordingly, the motor control device hashigh accuracy of estimating amounts of temperature rise. This enablesthe motor to continue operations without overheat. As a result, themotor control device can maintain high levels of both reliability andproductivity for job processing.

Even after stopping the motor in response to an estimated amount of risein temperature of the motor exceeding the threshold value, the motorcontrol device still continues to either measure the length of time thatelapses or estimate amounts of rise in temperature of the motor. Eitherwhen the length of the elapsed time exceeds the predetermined value Twt,or when an estimated amount Tr of temperature rise drops to the newthreshold value Tnt or below, the motor control device allows theprinter to restart job processing; the new threshold value Tnt indicatesthe temperature of the motor that can resume operation.

The motor control device thus, when the motor seems to escape the riskof overheat and become able to resume operation, automatically allowsthe printer to restart jobs. This enhances operability of the printerwithout impairing the ability to protect the motor from overheat.

Embodiment 7

A motor control device according to embodiment 7 of the presentinvention is mounted on a color laser printer, like the motor controldevice 310 according to embodiment 1. This motor control device differsfrom the motor control device 310 according to embodiment 1 only in partof the motor control procedure, and includes other elements similar tothose in embodiment 1. Accordingly, the following explains only thedifferent part of the motor control procedure, and incorporates theexplanation of embodiment 1 for the similar elements.

The motor control according to embodiment 1, when an estimated amount Trof rise in temperature of the motor exceeds the threshold value Tth,reduces the rotation rate of the motor. In contrast, the motor controlaccording to embodiment 7 first sets the threshold value Tth fortemperature rise amount to the upper limit of a range into which a fanincluded in the printer to cool the motor can reduce amounts of rise intemperature of the motor. The motor control changes rotation rates ofthe fan according to differences or ratios between the threshold valueTth and the estimated amounts Tr.

FIG. 14 is a flow chart of the motor control according to embodiment 7.This procedure differs from that illustrated in FIG. 7 only in stepsS141-S144 included instead of steps S106 and S107. Other steps in thisprocedure are similar to those in embodiment 1. Accordingly, thefollowing explains details of only steps S141-S144, and incorporates theexplanation of embodiment 1 for details of the similar steps.

In step S101, the rotation rate of the motor 330 is set to a targetvalue Ntg depending on operation modes. Then, the motor control proceedsto step S102.

In step S102, PWM control over the motor 330 is repeated several to adozen times. Then, the motor control proceeds to step S103.

In step S103, power loss of the motor 330 is measured from the dutyratio that the voltage instruction DTY indicates. For example, when stopof the motor 330 is detected from the duty ratio equal to zero, themeasured value is set to zero. Then, the motor control proceeds to stepS104.

In step S104, the LPF 332 uses the thermal model (5) of the motor 330 toestimate an amount of rise in temperature of the motor 330. Then, themotor control proceeds to step S105.

In step S105, the estimated amount Tr of temperature rise is checkedwhether or not to exceed the threshold value Tth. When the estimatedamount Tr is equal to the threshold value Tth or less, the motor controlproceeds to step S141. When the estimated amount Tr exceeds thethreshold value Tth, the motor control proceeds to step S108.

In step S108, the estimated amount Tr of temperature rise exceeds thethreshold value Tth, and thus the notification unit 313 has thecontroller unit 311 cut off power supply to the motor 330, and has themain controller unit 60 to suspend processing the jobs. In this way, themotor 330 is prevented from overheat. Then, the motor control ends.

In step S141, the estimated amount Tr of temperature rise is equal tothe threshold value Tth or less, and thus there has been no risk ofoverheating the motor yet. Furthermore, the notification unit 313 checkswhether the estimated amount Tr is equal to a first level or less. Thefirst level is equal to the lower limit of a range of temperature riseamounts that require increase in the ability of the fan to cool themotor, i.e. rotation rates of the fan; the lower limit is, for example,0.9 times the threshold value Tth. When the estimated amount Tr is equalto the first level or less, the motor control proceeds to step S142.When the estimated amount Tr exceeds the first level, the controlproceeds to step S143.

In step S142, the estimated amount Tr of temperature rise is equal to orlower than the first level, for example 0.9 times the threshold valueTth, and thus the rotation rate of the fan does not need to beincreased. The notification unit 313 further checks whether theestimated amount Tr is equal to a second level or less. The second levelis equal to the upper limit of a range of temperature rise amounts thatrequire reduction of rotation rates of the fan; the upper limit is, forexample, 0.6 times the threshold value Tth. When the estimated amount Tris equal to the second level or more, the motor control proceeds to stepS109. When the estimated amount Tr is less than the second level, thecontrol proceeds to step S144.

In step S143, the estimated amount Tr of temperature rise exceeds thefirst level, for example 0.9 times the threshold value Tth. Then, thereis a risk that the temperature rise amount reaches the threshold valueTth. Accordingly, the notification unit 313 has the main controller unit60 change operation modes to increase the rotation rate of the fan, andthus to further cool the motor or its surroundings. Then, the motorcontrol proceeds to step S109.

The notification unit 313 also has the measuring unit 331 correct ameasured value PL[·] of power loss according to the increase of therotation rate of the fan. This correction is necessary because of thefollowing reason. The increase in cooling ability of the fan increasesan amount of heat dissipated from the motor, and accordingly increasesan amount of heat generated by the motor, which is equal to an amount ofheat stored in the motor plus the amount of heat dissipated from themotor, i.e. power loss of the motor. In order to cause the measuredvalue PL[·] to reflect such an increase or decrease in power loss, themeasured value PL[·] may be multiplied by a correction coefficient moreor less than one, for example. Such a correction coefficient for eachdifferent rotation rate of the fan is determined in advance byexperiments or simulations, and is stored in a memory that is, togetherwith the measuring unit 331, embedded into an electronic circuit such asan ASIC and the FPGA.

In step S144, the estimated amount Tr of temperature rise is less thanthe second level, for example 0.6 times the threshold value Tth, andthus there is no risk that an amount of rise in temperature of the motorincreases to the threshold value Tth even if the fan has a reducedcooling ability. Accordingly, the notification unit 313 has the maincontroller unit 60 change operation modes to reduce the rotation rate ofthe fan, and thus to reduce power consumption and noise of the fan. Thenotification unit 313 also has the measuring unit 331 change acorrection amount for the measured value PL[·] of power loss accordingto the reduction of the rotation rate of the fan. For example, when themeasured value PL[·] is to be multiplied by a correction coefficient,the notification unit 313 has the measuring unit 331 to reduce thiscorrection coefficient. Then, the motor control proceeds to step S109.

In step S109, the controller unit 311 checks whether or not the maincontroller unit 60 has received an instruction to power off the printer100. If the instruction has been received, the motor control ends. Ifnot, the control is repeated from step S101.

Advantages of Embodiment 7

The motor control device according to embodiment 7 of the presentinvention estimates amounts of rise in temperature of the motor by usingthe thermal model of the motor, like the motor control device 310according to embodiment 1. Accordingly, the motor control device hashigh accuracy of estimating amounts of temperature rise. This enablesthe motor to continue operations without overheat. As a result, themotor control device can maintain high levels of both reliability andproductivity for job processing.

Furthermore, this motor control device changes the rotation rate of thefan included in the printer according to the difference or ratio betweenestimated amounts of rise in temperature of the motor and the thresholdvalue. The motor control device thus enables the fan to efficiently coolthe motor.

SUPPLEMENT

The motor control devices according to the above-described embodimentsof the present invention use the thermal model of a motor to estimate anamount of rise in temperature of the motor from the input values of themotor that the device instructs to the driver circuit of the motor. Thisthermal model involves both amounts of heat stored in and dissipatedfrom the motor, and thus the estimation unit can calculate estimatedamounts of rise in temperature of the motor even while it stops driving.In addition, the input values of the motor reflect feedbacks of statusesof the motor and driver circuit and load fluctuations. These devices canthus enhance the accuracy of estimating an amount of rise in temperatureof the motor regardless of repetition of intermittent drive of the motorand a variety of load fluctuations.

Based on the above-described embodiments, the present invention may becharacterized as follows.

The motor control device may further comprise a detector unit configuredto detect a rotation rate of the motor to be used as the output value ofthe motor, and a rate control unit configured to calculate a width of apulse from the difference in rotation rate of the motor between a valuedetected by the detector unit and the target value depending onoperation modes of the system, and to assign the width of the pulse asthe input value for the motor. The pulse is to be applied to the motorby the driver circuit of the motor. In this case, the estimation unitmay include an measuring unit configured to measure a power loss of themotor from a pulse width calculated by the rate control unit, and a lowpass filter (LPF) configured to integrate power losses of the motormeasured by the measuring unit with the thermal model of the motor toestimate the amount of rise in temperature of the motor. Alternatively,the estimation unit may include a LPF configured to integrate pulsewidths of the motor calculated by the rate control unit with the thermalmodel of the motor to estimate the amount of rise in temperature of themotor.

The estimation unit may update estimation values at longer timeintervals than the controller unit updates input values for the motor.Alternatively, the estimation unit may be equipped with two types of thethermal model of the motor, a heating model and a cooling model, anduses the heating model when the input value of the motor or the outputvalue of the motor indicates driving of the motor, and uses the coolingmodel when the input value of the motor or the output value of the motorindicates stop of the motor. In addition, the operation modes of thesystem may include two or more modes differing in average power loss ofthe motor, and the notification unit arranges an order in which thesystem should change operation modes to reduce the average power loss ofthe motor stepwise, and instructs the system to change operation modesin the order by sending requests for change of operation mode to thesystem.

The main controller unit of the image forming apparatus, in response tothe request for change of operation mode from the notification unit: mayselect an operation mode to be changed depending on the length of timebefore a job currently processed will be finished; may change operationmodes to enlarge the intervals of feeding sheets, to reduce the rate offeeding a sheet, or of two or more motors, to keep one motor waiting orto reduce the average driving time of the one motor when, for the onemotor, the value estimated by the estimation unit exceeds the thresholdvalue.

This image forming apparatus may comprise a fan configured to cool atleast one portion of the image forming apparatus, wherein the maincontroller unit, in response to the request for change of operation modefrom the notification unit, may changes operation modes to change arotation rate of the fan.

Although the present invention has been fully described by way ofexamples with reference to the accompanying drawings, it is to be notedthat various changes and modifications will be apparent to those skilledin the art. Therefore, unless such changes and modifications depart fromthe scope of the present invention, they should be construed as beingincluded therein.

What is claimed is:
 1. A device for controlling a motor mounted on asystem with different operation modes, comprising: a controller unitconfigured to calculate an input value for the motor based on an outputvalue of the motor and a target value depending on operation modes ofthe system, and to instruct a driver circuit of the motor to apply theinput value to the motor; an estimation unit configured to estimate anamount of rise in temperature of the motor by applying the input valuefor the motor to a thermal model of the motor; a notification unitconfigured to compare a value estimated by the estimation unit with athreshold value, and when the estimated value exceeds the thresholdvalue, to send a request for change of operation mode to the system, adetector unit configured to detect a rotation rate of the motor to beused as the output value of the motor, and a rate control unitconfigured to calculate the width of a pulse from the difference inrotation rate of the motor between a value detected by the detector unitand the target value depending on operation modes of the system, and toassign the width of the pulse as the input value for the motor, thepulse being to be applied to the motor by the driver circuit of themotor.
 2. The device according to claim 1, wherein the estimation unitincludes: a measuring unit configured to measure a power loss of themotor from a pulse width calculated by the rate control unit; and a lowpass filter configured to integrate power losses of the motor measuredby the measuring unit by using the thermal model of the motor toestimate the amount of rise in temperature of the motor.
 3. The deviceaccording to claim 1, wherein the estimation unit includes a low passfilter configured to integrate pulse widths calculated by the ratecontrol unit by using the thermal model of the motor to estimate theamount of rise in temperature of the motor.
 4. The device according toclaim 1, wherein the estimation unit updates estimation values at longertime intervals than the controller unit updates input values for themotor.
 5. The device according to claim 1, wherein the estimation unitis equipped with two types of the thermal model of the motor, a heatingmodel and a cooling model, and uses the heating model when the inputvalue of the motor or the output value of the motor indicates driving ofthe motor, and uses the cooling model when the input value of the motoror the output value of the motor indicates stop of the motor.
 6. Thedevice according to claim 1, wherein the operation modes of the systeminclude two or more modes differing in average power loss of the motor;and the notification unit arranges an order in which the system shouldchange operation modes to reduce the average power loss of the motorstepwise, and instructs the system to change operation modes in theorder by sending requests for change of operation mode to the system. 7.An image forming apparatus comprising: a main controller unit configuredto assign an operation mode depending on a job received from a user; twoor more motors configured to be used in transferring a sheet and formingthe image on the sheet; two or more driver circuits configured to supplypower to their respective motors of the two or more motors; and a motorcontrol unit including: a controller unit configured to calculate inputvalues for the two or more motors based on output values of the two ormore motors and target values depending on operation modes of the imageforming apparatus, and to instruct the two or more driver circuits toapply the input values to their respective motors; an estimation unitconfigured to estimate amounts of rise in temperature of the two or moremotors by applying the input values for the two or more motors to athermal model of the two or more motors; and a notification unitconfigured to compare a value estimated by the estimation unit with athreshold value, and when the estimated value exceeds the thresholdvalue, to send a request for change of operation mode to the maincontroller unit.
 8. The image forming apparatus according to claim 7,wherein the main controller unit, in response to the request for changeof operation mode from the notification unit, selects an operation modeto be changed depending on the length of time before a job currentlyprocessed will be finished.
 9. The image forming apparatus according toclaim 7, wherein the main controller unit, in response to the requestfor change of operation mode from the notification unit, changesoperation modes to enlarge the intervals of feeding sheets.
 10. Theimage forming apparatus according to claim 7, wherein the maincontroller unit, in response to the request for change of operation modefrom the notification unit, changes operation modes to reduce the rateof feeding a sheet.
 11. The image forming apparatus according to claim7, wherein the main controller unit, in response to the request forchange of operation mode from the notification unit, changes operationmodes to keep one motor of the two or more motors waiting or reduce theaverage driving time of the one motor when, for the one motor, the valueestimated by the estimation unit exceeds the threshold value.
 12. Theimage forming apparatus according to claim 7, further comprising a fanconfigured to cool at least one portion of the image forming apparatus,wherein the main controller unit, in response to the request for changeof operation mode from the notification unit, changes operation modes tochange a rotation rate of the fan.